Since your youngest is tall, I'd probably opt for one of the higher shell height seats, and a radian would fit the bill. Radians puzzle well facing opposite directions, I believe, so RFing, FFing (4yo), RFing should work. Do you have an angle adjuster for the 2yo? My concern would be that getting an infant angle with the radian might make your passenger seat unusable for awhile, since Toyota does not allow carseats to touch the seatbacks.
Eta: if you can get to a babiesrus or buy buy baby, you could try a narrow combination seat for the 4yo in between the two RFing radians. I wouldn't get a booster for a 4yo.
Last edited by mrosehughes; 01-28-2013 at 09:37 PM.