Its been forever since i played it, but I used a full party of 6. I belive it was a fighter, barbarian, thief, ranger, mage and cleric. Thief is a must for find traps, and you need some sort of healing to get by in most of the bigger fights. It maybe could be done with 4, but i think there would be a lot of akward situations where it became unnecesarily difficult to handle a situation because of either not enough raw damage output, or lack of some specific ability to handle various monsters.
As far a leveling goes, the game is pretty much designed around a 6 person party. Following the pathline from one area to the next will never leave you characters behind the curve from a level standpoint.