So total number of available companion characters is 6 and all of them are available in very early game part?
Yes, I read about this 4 player trick.
It seems to be asked by many users.
If such thing like 4 customized characters party is allowed in multiplayer, then I wonder why it should not be possible in single player, especially because it is not so hard progamming to add such simple feature.
I know that creators put a lot of work into their characters so would prefer that players experience their great work with it.
Flat characters in party might be less interesting for some people, but there are also RPG players who prefer to customize own party that fit their vision and story.
I guess custom party option will be available with later update.
If not, then probably modding scene will do that.