it's possible... my daughter was a little shorter than that but that age (2wks shy of turning 6) when she outgrew her Nautilus with an 18" torso.
it's not USUAL, but it happens. this is where having the frontier being such a tall booster becomes really really beneficial.
Now my oldest is more of the just generally mutantly tall... he's in the 105th percentile for height... he was 4' tall at about 4yrs old... he outgrew the REGENT and turbobooster highback at 6.5yrs, 5-stepped at 8yrs old at 4'9" and 100lbs, and is currently 12yrs old (with size 11shoes still... I have a pair to match his age for next year) and 5'7&1/2" (5'8 with shoes, 5'7 without, so I give him the middle number), 131lbs, and still growing fast.
However, we're now having to buy him SMALLER size jeans (but with longer legs) because he hasn't gained weight in almost 2yrs and has actually lost a little since he stopped taking Intuniv. (I'm not worried, he eats tons of healthy stuff and eats more than I do... my great-aunt's son was exactly like this at this age... thinned out to super thin and was really tall) I think honestly for him (not most kids) part of it is also that since we've gotten his cholesterol level (thank you lovastatin) at the normal level (it's been high since who knows... it was first tested when he was 4), he's been so much more active. Starting around age 7 I noticed he wasn't as active... I wonder what having all that bad cholesterol in addition to the good cholesterol being low was doing with his heart and if that was making him tired.
Anyway... Ruthie has been growing legs and at almost 4'3", she only has about an 19" torso (even though the way we have to raise booster height, you'd think it was more, but she's also gained weight and of course, that squishes the booster into the vehicle seat more and then her primary booster has such small space between settings)