Google started the Google Play Edition phones service a few years back. This program offered you to you buy a specific flagship phone of some manufacturers with pure Android installed. For example, there was a Google Play Edition of Samsung Galaxy S4. This S4 had the exact same specifications as the original S4 but the only difference was that it came with pure stock Android from Google, which means there was no TouchWiz installed in Google Play edition of Galaxy S4.
The good news is that Samsung Galaxy S4 Google Play Edition has got the Android 5.1 Lollipop update today. The weird thing is that the update version is still Android 5.1 not 5.1.1 as 5.1.1 is out in the market for few months now. Well, at least there is something available. The update is available OTA and Google has started to roll out to all S4 GPE devices. It might take some time to reach your device so you might want to wait for a day or two, before getting too impatient.