I prefer a 4 member party. Turn based combat (which is very much preferable to real-time w/ pause for a number of reasons) will be slow with bigger parties, especially if you throw animal companions, familiars and summons into the mix. Also, Larian has wisely chosen to opt for fewer companion all over with the trade-off being that each character will be considerably more fleshed out. Finally, a smaller party makes the party selection/character build decisions matter more. Larian has got this.