It's possible. At first you'll have to make your device's bootloader unlocked, this is the hardest step of all since the method to do this is different to each

It's possible. At first you'll have to make your device's bootloader unlocked, this is the hardest step of all since the method to do this is different to each variant of device and usually require root access. The second step is installing a recovery ROM which will allow you to install custom ROM. Finally, you find the custom ROM that suits your need and install it. Cyanogen Mod and AOSP provide almost if not the same as pure Google experience. Please note that S4 is very new so custom ROM may not be perfectly stable yet. You should find more information at forum xda-developers where developers are actively work on it.