Well first off a new case would help your situation and as an example the Cooler Master HAF-X case has 9 slots in the back of the case and will accomodate what you want to do very nicely.
Next the easiest thing to do for 4-way SLI is to get two 590's or 690's and that would give you 4-way with four slots.
Next is water cooling blocks on the video cards that would make them single slot cards. The Evga Hydro Copper 2 cards are an example of this and you can easily gat four of these cards in a 7 slot case. There are fittings that will connect the cards in 4-way single slot setup.
I have put a two slot card in the bottom slot and it was not an easy thing to do because you cover up the bottom row of connections , usb, fire wire start button pins and the card will just go past the last slot and as long as there is nothing to restrict it you caan fit the card.
My current case is the HAF-X and I had three hydro copper cards and a revo card in the four slots and could have easily put a fourth hydro copper card instead of the revo card in that fourth slot. The top two slots were unused because they lined up with the north bridge chipset so I was effectively using 7 slots. The maotherboard is a Gigabyte G1 Assassin MB.