Note, these mods only take into account companions. Unfortunately, a custom party +4 is not possible.
Huh. Well this is some terrible news I was not aware of as I was banking on some runs with 5 or 6 customs. Here's to hoping modders are able to make it work then because otherwise a huge draw of the game is gone, for me. I really wanted to run two parties of D&D concepts I've never gotten to play as due to just about always being the "forever DM" with my friend group.
Maybe once the full game is out and the whole mercenary/custom character idea is completely released!
So, you can only have 4 companions, but you can have extra followers. Swen discussed this in an old interview.
They also mentioned that there was a way to "recruit" defeated enemies as followers. Not sure how that system works.
Finally, as has been pointed out there are mods that will allow you to expand your party size if the Hirelings and the other available followers are not enough.
Also, since you are not party locked after Act 1 - we can switch out characters at will by returning to camp/stronghold.