I had this same problem when i updated my s4 software within a few weeks i was having white blocks instead of numbers or sometimes it was letters.  It also affected by pin lock numbers 
would you believe i read all the reviews but in the end i just turned my s4 off and back on nothing else i didn't reboot but i was happy when i switched my phone back on and everything was back to normal  (no blocks ) please try it before you do anything else