Ubuntu 14.04

UE4 will work on any linux as long as dependies are meet, most linux distros are the same. Linux support is heavily work in progress, current builds support cross compiling from windows but without editor. Theres huge community effort to make editor working on linux, i think if you search deep enoth you might already find how to make editor run on linux. Heres good starting point

Also theres dedicated irc chat #ue4linux on freenode, they might tell you exact status on linux support