heres on theory that helps:
how about this:
there is only one state of limbo (i think they go over this), which is shared by evreyone.
You can enter limbo by either 1. dying, or 2. going to deep into the dreams (or in this case, level 4).
fischer, saito enter limbo because they die.
cobb, ariadne, enter by going under again
cobb knows where to find fischer, because he knows that mal will be holding him hostage, and he knows where, because shes part of his sub-conscious.
they find fischer, ariadne and fischer exit limbo by killing themselves(which cobb has said is the only way to exit)
cobb then goes on a journey to find saito.. he walks. this is why he appears in the water exhausted/tired. he has to find him so that he can see his daughters again. in the time it takes him to find saito, a long time has past. since cobb is aware he's in limbo, he doesn't age, he just looks beatup from all the travel. Saito on the other hand, doesn't know he is in limbo, and is thus old as hell. cobb finally finds him, and wants to bring him back.
past that i have no idea