If all 4 characters have to be the same class, I certainly would go bard. I'll take the companions, I like to use the most, maybe Karlach, Shadowheart and Wyll or Gale.
If it is 4 characters with a single class, my Tav/Durge would be bard with high Dex, so that they can be the party rogue, then Karlach as bearbarian, Shadowheart as her usual cleric self ( subclass might change after a certain story part), Gale as our nuker with evocation.
I might change out Karlach for Lae'zel since she is more important for the main story - I like them both and they are both good. I also might change Gale for Wyll, since he, too, has more ties to the main story. Lae would be battlemaster fighter and Wyll bladelock.
Most of the companions stay single class in my game, I like Shadow and Gale to have their full caster levels. Wyll, I often level up as a paladin of vengeance midgame, because that fits him. Astarion is a character, I often respec, if I use him, since I like to have thieve skills on my main. And since he is not defined by his class, I just pick, what I need.
