I would personally introduce all four at the same time... it just makes more sense. If you're looking for tips on how to introduce them its really not that hard... just introduce them all on neutral ground (like a couch or mattress) that way they have leg room to explore each other, and also enough to room to run away if being attacked... Watch out for aggressive behaviour between the four... and if you notice them chasing each other around, its normal, they're just trying to smell each other out, anyway, Good luck
