I think a 4-char party *could* be fine. Currently I'm running Wizard, Wizard, Fighter, Cleric (MC, Gale, Lae'zel, Shadowheart) and I can pick locks and disarm traps just fine mostly. Still, I might go with a Rogue MC next time to make some things easier. (I can't stand Astarion. No chance.)

Like others already noted, 5e rules don't seem to be very rigid with regard to the roles of characters, so you can do lots of interesting things.

Of course I would still love 6 characters based on a few simple facts like:

1. It will feel more like the old Infinity Engine games.
2. More NPCs to chose from during each run, so you get to experience more of them per run.
3. Possibly more versatile and strategic combat as you have more characters to control.

However I'm not sure if it could be implemented well with the way the game looks so far. The following concerns come to mind:

1. The more characters in one party, the more interactions there can be, opening the gates for more bugs / logical inconsistencies in the way characters react to each other in dialogue. That said, the characters don't feel ALL that lively to me anyway right now (not sure if I've ever seen a character react to another character's reaction to my main character, if you know what I mean).
2. More characters to control means longer combat rounds. (You might say, but you will kill the enemy quicker. Well obviously there will have to be more enemies or beefier enemies to balance things. Hence longer fights.)
3. In some spots the characters are already at their limits regarding how well they can automatically follow your main character. Two more party members might make their movement very cumbersome and frustrating.

All in all, I'm undecided. It could be great theoretically, but the implementation might be very tricky/risky, ending up with more disadvantages.