I would do RF MA, FF RN, RF MA, and install the RF seat for the 2YO as upright as possible so 4YO can get into the middle seat :thumbsup: If that is still too wide, look at a RF RN instead of a RF MA, though that may be a difficult installation unless it's put in the middle.
This particular combination would be quite a bit too wide for my '96 Corolla; although I do realize newer Corollas are likely wider. I have to disagree about having the FF child in the center, in terms of ease of access. Installing an outboard RF seat more upright will not really give enough space for a person (even a small person) to slither through between the child seat backrest and the front vehicle seat backrest. Crawling through down closer to floor level is possible, I guess, but not terribly convenient, especially if the RF seat is tethered. I simply put the FF child outboard in this situation. Theoretically, of course (if relative safety is the
only thing you're considering) the FF child goes in the middle while the better protected RF kids go outboard; however, other factors have to be considered in real life. Examples of such factors are: access to seats, how the seats puzzle together, and how the children in the seats get along with each other in close proximity.
Oh, I do want to mention that the Radian and Marathon puzzle together very well when
both seats face the same direction -- the combined width of the two seats is almost 3" less than their individual maximum widths added together. So, RF Marathon, RF Radian, FF Marathon is more likely to work; also, this puts a RF Radian in the center, where it is more likely to fit (coming up between the front vehicle seats a bit, since it is a very long seat front-to-back). Also for reference, RF Radian does
not fit outboard in my older Corolla -- it is impossible to move the front vehicle seat forward enough to clear the carseat's backrest.
Here are three recent discussions about 3-across situations in Corollas. Admittedly they are mainly about older Corollas, but hopefully you will find some of the information helpful:
http://www.car-seat.org/showthread.php?t=47298&highlight=corolla
http://www.car-seat.org/showthread.php?t=46913&highlight=corolla
http://www.car-seat.org/showthread.php?t=44325&highlight=corolla
Katrin