Well I'd say the standard is WoW, and WoW is 5man. I'm not sure why they went with 4 tbh, it just makes very little sense since it's a fact there's always more DPS than tanks/heals, so the more you can cram into a group the better for queues and such. 5's a fine number too, 10's good for harder content and then for larger groups they could have done 25 or 20 or whatever.
Think it might be PS3 stuff tbh, they might have figured 8 was best to run properly in harder content and 10 was starting to drop too many frames or whatever, don't know. That or just they liked the idea of 4 characters since there's 4 archetypes(tank healer melee dps ranged dps).
Anyway it might change eventually I think. Maybe next expansion. I think the more classes they have, the better it'd be to increase group size so you're allowed a bit more free room to decide the classes you bring and stuff.