Samsung Galaxy S4 "Prevention Information" USB OTG
I did also read about this. KNOX seems to be blocking the communication to USB devices in some cases.

The Google Nexus 5 (LG) is also available now. Its pretty cheap for the features it offers. You should take a look at it before you swap you device. There is also a ROOT available for the SGS4 but it would break your warranty ofcourse:

This is where i read it

more about the root. only use the one from the xda-developers forum!