I've played 4 player co-op or matchmaking ever since pc gaming, and then from halo 2 with the same friends - countless games, if we cannot play as a 4 player fireteam this is a gamebreaker for me and my friends. I saw someone say that bungie designed it this way because from playtesting it would be too easy, yet on the beta, me and two other friends had another fireteam of 3 join us in the exact same mission and help us - so basically that arguement is blown out the water - plus it's supposed to be an MMORPG, not exactly that with only 3 player coop IMO. But when playing on the xbox one, I approached a friend in my fireteam and clicked the right thumbstick down, and then pressed the button which is 'three horizontal lines' (dunno what it's called, it was formally the 'start' button) it says "fireteam 3 of 6", so I am hoping that this is just a beta restriction. If not, it frelling sucks, especially as this seems to be an absolutely fantastic game. And as the OP states, make the game harder the more people in your fireteam, borderlands did this - AND rewarded you with better loot for your troubles.