My 4-year-old daughter is over 42" tall and was 36 lbs. when last weighed. I am hoping to find a booster seat for children under 40 lbs., since I have a need for one that could be moved easily between cars, especially short-term rentals. My state (California) does not have a minimum weight for using booster seats, so my concern is the minimum weight specified by manufacturers. All of the booster seats I've found so far have had a minimum weight of 40 lbs.
Have you looked at Britax Combination Harness to Booster (scroll down to see). They can be used from 25 lbs as a 5-point seat, and as a shoulder belt booster later. This means they are pretty tall.
This one, for instance, say it can be used from 30-56 inches & 25-90 lbs as a harness seat, and from 45-62 inches & 40-125 lbs as shoulder belt booster.
My guess is that you have looked at backless boosters only, since you wanted to move it easily between cars. However, if the car's LATCH/ISOFIX anchors are easily accessible, they this seat is VERY fast to install. We have travelled with it, and it was no big deal to install it in the rental.