I recently switched to the fourth slot for my 25-month-old. I had the same concern about the slots being so far apart. He is a little above the slots if his back is against the seat back, but he slides down a little, since I have to use the crotch strap position that is the furthest out (>30 lbs for our radian). I posted about this issue a little while ago. There is just no way he can use the third slots. The majority of folks thought he was fine with the fourth slots. If I remember correctly, there was even one person who was actually a cpst instructor who said she had used it that way when she moved her own child to the fourth slots.
Rebecca
K - 7.5, 51.5", 57 lbs, Monterey
A - 25 months, 37.5", 32.5 lbs, rf in a Radian65