I strongly prefer 6 characters.
4 characters is flat out boring, and was a major hang up in my ability to enjoy the NWN games and Tyranny. There are basically 2 optimal party comps for this type of RPG in a 4 person party, Tank, Tank, Heal, DPS or Tank, Heal, DPS, DPS. This is very, very limiting as far as strategy and tactics goes, and while I love DA:O I'm not going to pretend that my ability to set complex AI scripts and then just watch or ignore the battle (occasinally aiming spells or changing targets) as opposed to actually engaging in the monotonous attrition based combat didn't play a large part.
5 I can't pretend I have a whole lot of experience with, but I'll explain why I prefer 6.
With 6 characters, you as a player have more flexibility in both strategy and tactics. More party comps, more active effects on both enemies and allies, more useful formations. This not only increases the number of interesting strategies the player can create, but it also means the Devs can create more interesting encounters. For a more specific example, In 4 person parties formation is not a thing. Melee up front, ranged in back, don't stand in AOEs, it's no more complicated than that. In a 6 person party you can have actual front/middle/rear lines, which means not only do you have to figure out a good formation for your particular strategy, but the devs can do things like have large groups of enemies sheltering a mage who drops big AOEs on your formation and now you not only have to get out of the AOEs, but also figure out how to salvage your formation so can remain effective and so you you aren't exposing your middle/rear lines to the hoards of enemies.
