Yes I have also already extensively put together my favorite custom party, BUT no matter what I do, I am always missing at least one additional fifth character slot. -.- I'm not only about the subclass, but also the background, combat style and more.
I agree that with just four people, chances are that there are going to be some things that our party isn’t great at. But I actually like this, as it encourages me to think creatively about how to overcome those weaknesses and to tackle challenges in different ways in different playthroughs.
Of course, it would be frustrating in a badly designed campaign where there was only one way to achieve a goal, so if your party didn’t have the right skill you could be stuffed. But I think Larian have generally done a good job of making sure we have numerous alternative ways of completing quests or beating enemies, and it would need to be a very lopsided party indeed that didn’t have the skills to manage one of them.