Anyone who's saying that a four man party means that your restricted to player the cookie cutter party are simply not aware of the flexibility 5e offers, or are just not creative. In most my playthroughs I haven't used a cleric, and even when I do I don't use them for anything more than emergency healing, preferring to use them instead for actual combat (with in tabletop 5e they can really excel at). Why do you need a tank? I mean unless you're building something around the sentinel feat (which doesn't seem to be implemented yet), there's not even a viable way for them to maintain threat? Even when playing tabletop, the idea you have to have certain classes is self-limiting. Why have a rogue when you can have a ranger with the criminal background? Or a cleric when you can have a wizard with the magic initiate feat? One of the amazing things about 5e is that it doesn't restrict things to people playing that specific class, and so far that has transferred reasonably well into bg3 and I imagine it will continue to do so as more classes/races are added. I mean the 5e bard in itself can simultaneously fill the typical cleric, wizard and rogue roles of a classic party. That means one slot for your obligatory fighter, and 2 cool looking meat shields.
This doesn't even factor in if they end up adding multiclassing to the game, which adds so many more options.
While I wouldn't object to them doing a 6-man party, I think four is easier to manage, is more engaging in multiplayer (as from experience playing tabletop I think anymore than 5 players just detracts from the experience), and actually encourages people to have more creative character ideas and finding different ways of approaching the same solution. Also for single player, 3 AI companions having interactions is interesting but managing, with (hopefully) plenty of replayability options. 5 AI companions, I feel like that just gets messy.
I'm actually pretty hyped by the idea of having a 4-man drow ranger squad, and as this game stands I feel like there is very little that would stand in the way of such a team.