Unfortunately, there are countless S4 clones, and they’re often marketed in a sheisty manner regarding brand. Also, people who develop custom firmware are going to be far less likely to work on well-made custom firmwares for generic no-name brand clones.
Unless you’ve got something like the Feiteng clone, or another well-known branded clone, it may be difficult to find any custom firmwares (or your stock firmware if that’s what you’re looking for)
You could try needrom.com . Make sure you backup before trying any firmwares.
My best advice would be to backup and use Xposed framework and GravityBox. See the Android General sticky.
Atif Naser