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.

I have one myself and I don't know if that'll be possible. The problem is that different devices, sold by different carriers, require different radio drivers and so on. Take for example the S2 and S3s sold by AT&T and T-Mobile: they are marketed under the same name, they look the same externally but they are completely different model numbers.

In other words, I'm not sure Google's version of the S4 will be an exact copy of AT&Ts. I hope it is, I can't wait to get rid of Samsung's crappy version.