Well Cyanogenmod is the most profound custom ROM anyone uses. You can get Cyanogenmod for your S4 from the Cyanogenmod website. [Google around you'll find something.] Root your phone, unlock boot loader, install CWM or TWRP recovery and then install Cyanogenmod and you're good to go.
P.S. - Try to use a stable and not a nightly build because nightlies can have lots of bugs yet to be fixed. But if you want to try Kit Kat on your phone ASAP, you can install the CM 11 nightly.