The GTX 780 does not support quad SLI. You can only SLI three cards max.http://forums.evga.com/Gtx-780-quad-sli-m1971498.aspx
But I wouldn't worry if I were you, because you will hardly get any performance out of the fourth card. It will only eat power and generate heat, possibly make more compatibility problems.
If you didn't already know, running multiple cards does not increase performance by 100% per card. In games that support SLI, a second card will add about 40-60%, and the third card no more than 30%. And there are many games that don't support SLI at all...
Put the two liquid cooled cards on the top two slots. That way the air cooled one gets all the air it needs.