Quad Combo is an Achivement that can be obtained when you suck up four ghosts all at once. It doesn't count if you do it in the Training Room, as if you could, this achivement would be way too easy. This is easily obtained, as you can "Gather" ghosts into an area, and defeat them all at once.
This Luigis Mansion Character Creator Wiki article is a stub.
Please expand it, or the Ghosts will get you!