You only need three to get through this wall - two open the door, one runs through. One of the door openers disengages and the person that ran through activates that side of the door from the other side of the wall. Then the disengaged players runs through, and finally the two players on the far side of the wall open it for the remaining player.
Which is completely academic, since you do need four real people for the final element of the heroic. One to throw the switch that lowers the forcefields. Two (one on each side) to use their Ghostbuster beams on the cores, and one to throw the grenade that destroys the cannon.
I can confirm that this is infact right. I do have to say though I was rather dejected after getting through the entire instance (as well as killing the boss) only to find that we actually needed 4 players to finish the quest.
Was very surprised at the lack of any warning (upon entering the instance) indicating this to be honest.