Diono Monterey 4DXT vs Cosco Topside

Both are booster car seats — here are their published manufacturer specs side by side.

This compares published manufacturer specs — not safety or fit advice. The seat's manual, your vehicle's manual, NHTSA's tools and a certified CPST are the fit-and-safety authority; always check them before installing.

How they differ

• the Diono Monterey 4DXT is listed narrow (roughly ≤17″) for fitting three across a back seat; the Cosco Topside isn't listed narrow
• the Cosco Topside is the lighter, more travel-friendly model

Published limits

Diono Monterey 4DXT: Belt-positioning booster convertible between high-back and backless mode (identical weight/height requirements printed for both — no per-mode split found in manual). No age requirement printed. Expired over 8 years from date of purchase (printed number, per manual).

Cosco Topside: Backless booster only (no high-back mode). No age requirement printed (weight/height/ear-position only). Do not use more than 10 years after the date of manufacture (printed number, per manual).

Printed verbatim in the manufacturer's manual (Diono Monterey 4DXT 2-in-1 High-Back Booster Seat Instruction Manual · Cosco Topside Backless Booster Seat Owner's Manual); “—” = not published for that mode, “not yet sourced” = we haven't read that manual yet, not that the seat has no limit. How we source these →

Recall record

Diono — 2 on NHTSA's record · Cosco — 21 on NHTSA's record.

That is the record of the brand — not a verdict on these models. A recall names specific models and manufacture-date ranges, and a seat missing from the record is not "recall-free": check nhtsa.gov/recalls with the model and date of manufacture on your seat's label. Source: NHTSA, retrieved 2026-07-16.

Where to buy

Check the Diono Monterey 4DXT price on Amazon →

Check the Cosco Topside price on Amazon →

As an Amazon Associate we earn from qualifying purchases, at no extra cost to you. Affiliate links never affect our specs or which seats we compare.

FAQ

What's the difference between the Diono Monterey 4DXT and the Cosco Topside?
The Diono Monterey 4DXT is narrow for 3-across; the Cosco Topside is lightweight/travel. Both are booster car seats.

Which fits three across, the Diono Monterey 4DXT or the Cosco Topside?
The Diono Monterey 4DXT is listed narrow (~≤17″) for 3-across; the Cosco Topside isn't. Three-across fit still depends on your exact vehicle — confirm before buying.

Which is right for my car and child, the Diono Monterey 4DXT or the Cosco Topside?
That depends on your specific vehicle and child — this page compares published manufacturer specs only, not fit or safety. Check the seat manual, your vehicle manual, NHTSA's car-seat finder and ideally a certified CPST before deciding.

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