A team of four is standard in multiplayer games. There is a fairly simple reason for this, it's hard to find more players. Another thing is that a lot of arenas are completely not adapted to more characters and will end up being extremely tight. Of course, if you increase the number of characters, you must also increase the number of enemies.
As for the intellect devourer they had to be weakened, I can't imagine fighting them on level 1 if they had instant kill ability (instant kill abilities is a bad project). Intellect devourers are quite famous monsters that are associated with mind flayers, so putting them in the prologue makes perfect sense. Changing opponents to match the campaign you want to launch is not unusual or even wrong.
