can I have more than 4 people?

[AsuraStrike]

It looks like there are 6 in total including the main character. I encoutered the 5th one but the party seems full... Can I recruit him anyway and swap in/out? If I have to, how do I kick current member out?

[Alive]

Party size is capped at 4.

Talk to current party members for a dialogue option to have them leave, freeing up space for who you want.

There's a mod that increases the party size.

Nlt sire what you mean by 'main character'.
There are 6 possible party NPCs not counting your own custom character.
Edit: guess I shouldn't call them NPCs since they're actually playable... but you get the idea.

[Neiker]

you have to choose only 3 companions. Its not a good idea to keep changing them, you'll see why on act 2.

Only way of getting a bigger party is with mods.

[ZyferTV]

what are the mods?

[Danson]

Originally posted by ZyferTV:

what are the mods?

Gonna take a wild guess that one is called something like "extended party" or "party size ____". Just a hunch.

[Pyromus]

Originally posted by ZyferTV:

what are the mods?

There was an older one before the definitive edition that (at least the last time I tried it) made some unintended side effects make the game interface unplayable. Search "extended party size" and "party size evolved" and whichever one has a more recent date on it is the one you want.