If you can fit 4 18's in that van, with proper airspace and the right box, it will be louder then the btl for sure. I'd go for the btl, like someone mentioned just for having one sub. But if you got the room go for it.
aye im pretty much decided on 4 18's i seen a picture of a guy with 4 in a chevy astro, the box sits 3-4 inches above his window line i was really hoping i could keep everything below.. the window line so i gotta play around and see