I wouldn't put the four year old in a booster. He's too young and too light. Four years and 40 pounds is the absolute bare minimum. Besides, the Turbobooster isn't very narrow.
What convertible is he in now? That's somewhat important, and more importantly, where are his shoulders in relation to the top slot? Are they over, under, just at the top slot?
I would probably get him a Radian and put him in the middle if you can. Then put the AO rear facing on one side and the Snugride rear facing on the other. Though the AO isn't terribly narrow either. I just don't think you're going to get a Radian rear facing in there. It's too big front to back.
As the baby gets older and you need a convertible in that location a Scenera would be a good idea. It rear faces to 35 pounds, like your AO, and then it forward faces to 40 pounds with 14.5" top slots (I hear new ones are a little higher). So it's not going to be the only harnessed seat you need, but it's $45 and quite narrow. And it's good for small babies.
That'd probably be my first attempt. Rear facing AO for the two year old, forward facing Radian for the four year old, then rear facing Snugride, and then Scenera, for the baby. That way the least protected child (forward facing) is in the most protected location. Since rear facing is five times safer than forward facing, and rear facing seats react better during side impact collisions than forward facing seats, I'd definitely want the two rear facing kids on either side.
If AO/Radian/Snugride (Scenera) doesn't fit, I'd get another Scenera for the two year old. Try that. Or, since you'll have a Radian you can try it out rear facing behind the passenger or driver (and in the middle as a last resort if you'd need to put a rear facing child there). Maybe it will fit, I don't know. If it does then you can get another one of those for the two year old. Then forward facing four year old, and rear facing baby. If you need to stagger out the seat costs I'd buy the Radian first, then in several months the Scenera (or buy the Scenera sooner for the two year old if AO/Radian/Snugride doesn't fit), and then buy a second Radian down the line if you need it.
HTH
Wendy