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.
Spec
Diono Monterey 4DXT
Cosco Topside
Stage
Booster
Booster
Narrow (3-across)
✓
–
Extended rear-facing
–
–
FAA-approved
–
–
Lightweight / travel
–
✓
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
Mode
Diono Monterey 4DXT
Cosco Topside
Booster (with back)
40–120 lb · 38–63 in
—
Booster (backless)
40–120 lb · 38–63 in
40–100 lb · 43–57 in
Useful life
8 yrs from purchase
10 yrs from manufacture
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).
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 2026-09-05.
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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.
Citing these specs? Go ahead — published manufacturer data, last verified 2026-06-11. Copy a ready-made reference: