The issue (already mentioned several times in this thread, I assume) is that a party of four somewhat locks your party composition choice.
You need a frontline, a buffer/healer, an arcane caster and then you have a free slot.

So you generally have to choose between bard/rogue/warlock or any niche spec. Having a party of 5+, on the other hand, grants you way more freedom and nuance in your party composition.

Oh well, I suppose it's too late now, Larian did not listened/delivered on this matter in a two years, I seriously doubt they will in the next couple of months.