[Phantom Lord]

Four member parties work fine and level up fast. And they're great to play because you don't have to manage six chars.

I'd strongly suggest to have at least one heavy duty tank in the party (Mazzy comes to mind). Since BG2 misses a good aligned wizard equivalent for Edwin, the spellcaster issue is the most difficult to decide. Aerie could prove a good choice, since she would also fill the cleric role you could think about Haer Dalis (more spells), Minsc or keldorn (both more brute force) as the fourth character.

With a dc wizard slayer / thief npc (probably awful strong combination at high levels) and good alignment I would go for Mazzy / Keldorn / Aerie. Having only one spellcaster means certain difficulties in battle preparation or battle itself, but I'm rather the "brute force" type player, so this is just a personal opinion, not necessarily a suggestion for the strongest possible party.