50% OFF Launch Offer: $169.99 (Reg. $339.99) · Free Tracked Shipping · Ships in 24h
Claim Offer ❯
Excellent4.8 (247)Trustpilot

Real Lumbar Support. Zero Wasted Floor.

Cosari SpineFlex M4

A dense, waterfall-edge seat that doesn't flatten, a ribbed mesh back that flexes with your lower spine, and padded arms.

Claim 50% Off — Order Now

In Stock – Ships Within 24 Hours

  • 3.4-inch molded seat cushion that doesn't sag
  • Slotted lumbar brace that flexes with your back
  • 90° flip-up arms that tuck under any 22-inch+
  • Silent, hardwood-safe dual-wheel casters
Cosari SpineFlex M4
Free Tracked Shippingin 5 to 7 business days
30-Day Home Trialtry it risk-free at home
Pay with PayPalVisa, Mastercard or Klarna
24-Hour Supportfast, helpful customer care

Built for the Way You Actually Work From Home

Four details that fix the most common reasons people return office chairs.

Slotted Lumbar Brace

Built into the frame behind your lower back. The slots let it flex as you move, so it supports the curve of your spine without a hard plastic ridge pressing in.

90° Flip-Up Arms

Padded arms rotate straight up. Slide closer to your keyboard, sit cross-legged, or roll the chair fully under your desk and get about 14 inches of floor back.

3.4-Inch Molded Cushion

52 kg/m³ high-resilience foam with a rounded waterfall edge. It holds its shape for months and keeps pressure off the backs of your thighs.

Ribbed Breathable Mesh

Horizontally ribbed elastomer mesh lets air move through the back, so you stay cooler through long afternoons than you would in leather or vinyl.

How the SpineFlex M4 Works

Your lower back is supported automatically

Your lower back is supported automatically

Sit back and the ribbed mesh and slotted brace meet the curve of your lower spine. There are no knobs to set. The brace flexes as you shift, so support follows you through the day.

Arms flip up, chair rolls away

Arms flip up, chair rolls away

Lift either armrest and it rotates to vertical. With both up, the chair slides fully under your desk apron, clearing the walkway when work is done.

Set your height and tilt in seconds

Set your height and tilt in seconds

The right-side lever adjusts seat height and locks the backrest upright. The knob under the seat sets how firmly the chair rocks between 95° and 120°.

Cosari SpineFlex M4 hero
Excellent4.8 (247)Trustpilot

Limited-Time Direct Offer — Save 50% Today

Cosari SpineFlex M4

Mid-back mesh chair with a 3.4-inch molded seat, a slotted lumbar brace, and 90° flip-up arms that tuck it under your desk.

Backed by our 30-Day Money-Back Trial & Full 2-Year Cosari Warranty.

$169.99 $339.99 Free Tracked Shipping (5–7 Days) Included

In Stock: Delivers in 5–7 Business Days. Free insured US delivery with tracking.

30-Day Money-Back Guarantee plus a full 2-Year Warranty.

Quantity

1
Add to Cart
PayPalVisaVISAMastercardKlarnaKlarna.
Secure Checkout5–7 Day Tracked Delivery30-Day Trial
Description

Description: The Cosari SpineFlex M4 is a mid-back ergonomic mesh chair designed for home offices. It combines a horizontally ribbed elastomer mesh back with an integrated slotted lumbar brace, a 3.4-inch waterfall-edge seat in heather-grey woven fabric, 90° flip-up padded armrests, a rocking tilt with tension control, an SGS Class-3 gas lift, a curved 5-star base, and five silent dual-wheel casters. The frame is finished in light silver-grey to suit lighter, home-style rooms.

Fit & Sizing: Seat height adjusts from 17.7 to 21.5 inches, fitting most users from about 5'2" to 6'1". The seat is 20.1 inches wide and 19.3 inches deep. With arms down, armrest height runs from 25.6 to 29.4 inches. With arms flipped up, the chair clears any desk 22 inches or higher. Rated for 300 lb.

Shipping & Returns: Free tracked shipping on every order. You have 30 days from delivery to return it for a full refund if it isn't right for you, plus a 2-year warranty on the frame, cushion, and gas lift.

5–7 Day Shipping & 30-Day Returns

Free 5–7 Day US Shipping: Every order ships free with full tracking and delivers in 5 to 7 business days.

30-Day Trial: Try it at home for 30 days risk-free. Need help? Email us anytime at support@buycosari.com.

2-Year Warranty: If anything goes wrong within 2 years, we replace it free of charge or refund your order in full.

Why Choose the Cosari SpineFlex M4?

Cosari SpineFlex M4 Detail

SGS Class-3 Gas Lift

Certified and tested to 120,000 cycles with a 300 lb rating. It holds your height without slowly sinking mid-call.

Silent Hardwood-Safe Casters

PU-coated dual wheels roll under 32 dB and won't leave marks on hardwood, laminate, or LVP. No floor mat needed.

12-Minute Solo Setup

Pre-aligned bolt holes, a labeled bolt card with a spare of each size, and one T-handle hex key. One person can build it in about 12 minutes.

Cosari SpineFlex M4 vs. Ordinary Alternatives

Cosari SpineFlex M4Standard Competitors

FeatureCosari SpineFlex M4Standard Competitors
Seat cushion3.4-inch, 52 kg/m³ molded HR foamThin sponge foam that flattens in weeks
Lower-back supportSlotted brace that flexes with your spineHard plastic block or none at all
Desk fitArms flip 90° and tuck fully under the deskFixed arms that hit the desk edge
Floors & noiseSilent PU-coated casters, no scuffsHard plastic wheels that mark and rumble
Look at homeLight silver-grey, compact mid-backBulky black office frame
Order Now & Save 50%

What's Included

In the Box
  • 1 × Cosari SpineFlex M4 chair (mesh backrest, seat cushion, 2 flip-up armrests, 5-star base, gas lift, tilt mechanism)
  • 1 × Set of 5 dual-wheel casters
  • 1 × T-handle hex key
  • 1 × Labeled bolt card with one spare bolt per size
Technical Specifications
  • Seat height: 17.7–21.5 in (floor to top of cushion)
  • Seat size: 20.1 in wide × 19.3 in deep
  • Cushion: 3.4 in, 52 kg/m³ high-resilience molded foam, woven fabric cover
  • Armrests: Padded, 90° flip-up; 25.6–29.4 in high when down
  • Tilt: Rocking 95°–120° with tension knob; locks upright
  • Gas lift: SGS-certified Class 3, tested to 120,000 cycles
  • Capacity: 300 lb static
  • Base & casters: 25.2 in curved 5-star base, 5 PU-coated dual-wheel casters
Care & Safety
  • Vacuum the woven seat with a soft brush attachment once a week. For spills, blot with a damp cloth and mild soap, then let it air dry. Don't soak the cushion.
  • Wipe the mesh and frame with a dry or slightly damp microfiber cloth. Every few months, check that all bolts are snug and clear hair or dust from the caster wheels.
  • Backed by our 30-Day Money-Back Guarantee & 2-Year Warranty
  • Customer Support: support@buycosari.com
In the Box Cosari SpineFlex M4

Built and Ready in 3 Steps

Swipe left for steps 2 and 3

  1. Press in the casters and gas lift
    1

    Press in the casters and gas lift

    Flip the base over, push the five casters into place until they click, and drop the gas lift into the center. No tools needed.

  2. Attach the back and arms
    2

    Attach the back and arms

    Line up the pre-drilled holes and tighten each bolt with the included T-handle hex key. Everything fits without a second person.

  3. Sit down and set your height
    3

    Sit down and set your height

    Seat the chair onto the gas lift, sit down, and pull the right-side lever until your thighs are level and your feet rest flat.

Verified Customer Reviews

What 200+ Verified Buyers Say About the Cosari SpineFlex M4

Excellent Trustpilot
4.8
Based on 247 verified reviews
5 Stars
212
4 Stars
25
3 Stars
7
2 Stars
2
1 Star
1
97% of buyers recommend the Cosari SpineFlex M4
Filter by topic
Customer photo
✓ Verified Purchase
Daniel K.Verified Buyer · 4 days ago

Finally clears my 27-inch desk drawer—and zero lower-back lockup

My spare-bedroom desk has a low 27-inch drawer apron and every chair I owned banged straight into it until this one. I keep one arm flipped up when I want to pull in close or play acoustic guitar between meetings, and at 5:30 PM I flip both arms vertical and push it completely flush under the desk. Reclaimed over a foot of walkway and my lower back hasn't stiffened up once.

Verified Purchase · Austin, TX · Cosari SpineFlex M4

Customer photo
✓ Verified Purchase
Rachel M.Verified Buyer · 1 week ago

Built it by myself on my lunch break in 12 minutes flat

Put the entire chair together solo right next to our dining table in about 12 minutes—every single bolt lined right up on the first try without needing someone else to bend the armrests into place. My last budget chair felt great for a week before the foam died and my tailbone hit the board underneath. Three weeks into daily 9-to-5s on this 3.4-inch woven cushion and it hasn't compressed a millimeter.

Verified Purchase · Columbus, OH · Cosari SpineFlex M4

✓ Verified Purchase
Greg H.Verified Buyer · 1 week ago

No more bracing on my desk after 3-hour client calls

Before switching to the M4, I stood up from a 3-hour client call and literally had to brace both hands on my desk for ten seconds before my lower back would unlock. Other ergonomic chairs have that hard plastic lumbar block that feels like two fists digging into your kidneys, whereas this ribbed mesh and slotted brace actually flex with your spine instead of fighting it.

Verified Purchase · Denver, CO · Cosari SpineFlex M4

✓ Verified Purchase
Lauren S.Verified Buyer · 2 weeks ago

Beat the $329 DTC chair I returned last fall

Spent $329 on a heavily advertised direct-to-consumer chair last year and sent it back because the seat felt like sitting on a carpeted cinderblock by 2 PM. Grabbed the SpineFlex M4 at 50% off and the waterfall edge makes a massive difference behind the knees. Breathable back, thick padded seat—best of both worlds.

Verified Purchase · Raleigh, NC · Cosari SpineFlex M4

✓ Verified Purchase
kevin p.Verified Buyer · 2 weeks ago

Silent on Zoom calls and safe on white oak floors

Zero creaks, zero metallic clicks when leaning back on quiet client Zoom calls, and the dual casters glide over our white oak hardwood floors like butter without leaving black scuff marks. Also love the light silver-grey colorway—it doesn't make our guest room look like a 1998 insurance cubicle.

Verified Purchase · Seattle, WA · Cosari SpineFlex M4

✓ Verified Purchase
Melissa T.Verified Buyer · 3 weeks ago

My physical therapist was right about the waterfall seat

My PT told me one proper waterfall-edge chair with L4 lumbar support would do more for my sciatic flare-ups than ten dry-needling appointments. She wasn't kidding. Two weeks in and the tingling down my right hamstring at the end of the workday is completely gone.

Verified Purchase · Chicago, IL · Cosari SpineFlex M4

✓ Verified Purchase
Brandon W.Verified Buyer · 3 weeks ago

Woven foam seat > full mesh cheese-grater seats

I learned the hard way that full-mesh seats act like a cheese grater on sweatpants and pinch under your thighs. Having a dense woven foam seat paired with an open ribbed mesh backrest keeps me cool in an Arizona upstairs office without sacrificing tailbone cushioning.

Verified Purchase · Phoenix, AZ · Cosari SpineFlex M4

✓ Verified Purchase
Trevor B.Verified Buyer · 1 month ago

Rock-solid work chair (just note it's a true mid-back)

I'm 6'1" and 210 lbs. If you want a tall headrest to lean your neck back and watch movies, keep in mind this is a mid-back task chair. That said, for actual upright coding posture, it locks my lower back right into pocket, the gas lift has zero wobble, and the seat foam holds my weight effortlessly for 8+ hours.

Verified Purchase · Minneapolis, MN · Cosari SpineFlex M4

✓ Verified Purchase
Hannah L.Verified Buyer · 1 month ago

My living room stops looking like an office at 5 PM

Working out of a 700-sq-ft apartment meant a giant black gaming chair was completely out of the question. The silver-grey mesh blends right in with our sofa, and flipping the arms up lets me slide the whole chair flush under a narrow Parsons table every evening.

Verified Purchase · Boston, MA · Cosari SpineFlex M4

✓ Verified Purchase
Nina G.Verified Buyer · 1 month ago

Cross-legged sitters: get the flip-up arms

I switch between sitting normally on video calls and sitting cross-legged while editing spreadsheets. Being able to pivot both padded armrests straight up gets the plastic completely out of my way, and the wide woven cushion has plenty of room without hard side bolsters digging into my hips.

Verified Purchase · Portland, OR · Cosari SpineFlex M4

Try It at Your Own Desk for 30 Days

$169.99 $339.99 Free Tracked Shipping (5–7 Days) Included

Claim 50% Off — Order Now

In Stock – Free 5–7 Day US Shipping · 30-Day Risk-Free Trial.

Frequently Asked Questions

Will the SpineFlex M4 fit my height?

The seat adjusts from 17.7 to 21.5 inches off the floor, which fit our testers from 5'2" to 6'1" comfortably. If you're over 6'3", you'll likely want a taller chair with a headrest.

Will it fit under my desk?

With the arms down, the armrests sit between 25.6 and 29.4 inches high. Flip them up 90 degrees and the chair clears any desk with at least 22 inches of space underneath. It tucked fully under our 27.5-inch desk apron.

Is it safe on hardwood or LVP floors without a mat?

Yes. The five dual-wheel casters are PU-coated, and they left no marks after 500 loaded rolls across bare, unsealed red oak in our testing.

What's the weight capacity?

It's rated for 300 lb static load, with an SGS-certified Class-3 gas lift tested to 120,000 cycles.

Does the backrest lock, or does it always rock?

Push the lever in and the back locks upright. Pull it out and the chair rocks between 95° and 120°, with tension set by the knob under the seat. It doesn't lock in reclined positions.

What if it doesn't work for my back?

You get a 30-day money-back guarantee, so you can work full days in it at your own desk. If it isn't right, return it for a full refund. Structural parts and the gas lift are also covered by a 2-year warranty.

Cosari SpineFlex M4In Stock · 30-Day Trial
Order Now & Save 50%