You should follow the advice of what water coolers are telling you.  Don't do it unless you're fine with throwing away money and know what you're doing (or are willing to spend a bit of time learning).  It's not going to be easy/cheap to cool several of these cards.

Whether you should go for 3 or 4 cards depends on whether or not you want to overclock them.  With 3 overclocked cards, you can easily approach the same power usage as 4 cards at stock speeds.  With 4 slightly overclocked cards and some memory underclocking, you could be hanging around 1800w, depending on how good the PSUs are.  You'd want two PSUs to do 4 cards.  With 3, you're pushing it with a single PSU and should go for 2 PSUs.  You might be looking at 900-1300W just for the cards themselves.  You'll want to stay under 1440W total draw on a 120V/15A breaker, or 1920W on a 120V/20A breaker to be on the safe side.  3 cards would be safer and cost less.  4 cards is a fire hazard.