It’s a bug with Samsung’s build of Android 4.3 (since they have to “adapt” Google’s vanilla version of Android to their S4 hardware).
Before I bought my S4 Google Play Edition, I had a Nexus 4 which had a perfect working Spell Checker... weirdly all the settings are exactly the same in my S4 GE, but the spell checker just doesn’t seem to work at all as it does in my Nexus 4.
I just hope Samsung fixes this major bug (along with the ability to read/write again to our SD card) with the release of KitKat.
You can read more on this here:
http://forums.androidcentral.com/sam...galaxy-s4.html