Booster seat with highback

My daughter is 4. Right now we have her in the high back booster with the harness. She is within in the height of her age group, however, she is very thin and only weights 30 pounds. Can we use the seat belt instead of the harness? I've been trying to find information regarding this but can't.

As long as her shoulders are still below the harness slot, I'd keep her in the harness and not the seat belt. 30 pounds is still so small, and she's so much safer in a 5-point harness.

Graduating kids into boosters from harness (not to mention RF to FF) before they reach the maximum weight & height limit increases their risk of injury.... Safest practice is to keep big kids in the booster until they no longer fit, regardless of age, according to the owner's manual - to keep "middle sized" kids in the harness as long as possible - to keep infants/toddlers rear facing as long as possible.

When FF the kids' shoulders must be even with or below the harness slot, so if your daughter's are then keep her in the harness. Your daughter could technically ride RF in the kind of convertible seat we have (since when RF the shoulders must be even with or below the harness slots) & there are quite a few seats now with higher FF harness weight/height limits ... maybe after viewing the FF photo album of kids over 40# you & your daughter might be interested in a seat that harnesses much longer - as you'll see below, my 7 SD still rides happily in a harness seat She prefers it over her booster.

At the very least, when the time comes to switch from harness to booster, you might look at other boosters that offer more in the way of comfort & protection than a combination seat without head wings - like the Britax Parkway or Graco Turbo (which can be used backless for emergencies or travel)....