I have no idea if Larian knows how much exp they are dishing out in EA right now.
Even if they didnt back when EA was released ... i bet they do by now.
Becoming level 5 or even level 6 in Act 1 would upset the balance so much that encounters become trivial.
Unless those encounters will be ballanced for that speciffic level.
But you probably mean rest of encounters in Act 1 ... well, that is where XP adjusting comes to play.
Limiting or removing exp gain at level 4 is a bad way to compensate for this as it discourages exploration of side content.
You missunderstand me ... maybe its my bad english.
I never presumed that there will be some level cap for each act ... it would be a way, to be sure ... but certainly not good one.
I presumed that they will take all XP you can get in Act 1 and adjust them to fit the model.
For example: (dunno the numbers, will just guess)
You need 500xp to become level 1, 1000xp level 2, 2000xp level 3, 4000xp level 4, 8000xp level five.
So ... if they want us to be level 4 at the end of Act 1 ... all they need to do is make sure that whole amount of XP in Act 1 will be moving between 4000 and ... dunno, for example 6000.
That way we are sure to stay at level 4, and also are quite sure to reach it even if we miss something.
Right now the whole amount of XP can be even 9000 ... and that is exactly what they need to tune up.