Apparently it's the full six pokemon over wireless and FC battles- 4 pokemon is only on random battle.Boo.
That's quite a shame.
Speaking of that fourth Pokemon, that seems like an important role; it's your only "surprise" option available.
I could see a backup weather-starter, Eccentric Ditto, and another sweeper (after a bait set on something else eliminates its threats) all being useful there.
Also, shame about Regigias. But I wouldn't count him out just yet- Regi's biggest weakness has always been that he can rarely get Slow Start to run out before he is forced out. However... With rotation, Substitute, and a Wishing partner... he has a chance at glory.