You're gonna have fun with 4 980Ti's
OT: I don't have experience with quad-sli setups, but i had a tri-Xfire setup a while ago which is somewhat the same.
What i did was, i made a hole in the side panel of my case and mounted a fan that blew cold air onto the cards so that they pulled in cold air. This greatly reduced my temps.
A lot of cases already have fans mounted on the side. So just take a look at it.
Also, check how all the other fans are performing,...