Probably water cooling. For reference, I have 3 6990s on an open-air rig sitting directly below a large AC unit and it takes two Ultra Kaze fans blowing directly on the chips to keep them at 80 degrees Celsius. I don't think 4 cards would last very long in an enclosed case, personally, but they might do alright if you spread them out, give each one its own fan, and somehow turn the whole case into a veritable jet-intake without giving up that "regular computer" look.

Thing is, you probably won't be able to spread them out that much. PCI risers only give you about 6 inches, which sounds like a lot but really, in practice, isn't so much. Especially since you have to keep slack in them to keep from pulling them out of their slots, and since you can't twist them more than maybe 30 degrees at most without losing significant length.

P.S.
If your cards are anything like my 6990s, even if you manage to make the case look *normal* while incorporating awesome ventilation, the sound's going to give it away. And the heat. "Hey, why does your computer sound and feel like a hair dryer? You wouldn't happen to be mining, would you?"

Yeah. For stealth, water cooling all the way.