if I remember correctly even the old bgs were a maximum of 4, but I could be wrong. Honestly with a group of 4 there is a lot more strategy and I personally like it. Six would be too much, they should increase the difficulty of the mobs and should increase the mobs to face.
The origional Baldur's Gate games both had 6 man parties as the default party size when you was done recruiting companions, as did the Icewind Dale games that where built on the same engine. If they where all based around a 4 man party then this wouldn't be such a big deal to all of us who loved the originals.
Same for Torment, while Temple of Elemental Evil had a default party of 5 player-generated characters but you could hire up to three additional NPCs, if charisma allowed it. Fun fact: ToEE had for years what was broadly considered as by far the best, most faithful implementation of the D&D combat system in a computer game (its flaws were all in other areas, frankly) and allowed for a bigger party *precisely* because turn-based combat made it more manageable and rewarding than real-time.