I had the same issue - navigation bar was untochable in any position (vertical & horizontal). Back, home nd recent buttons were 100% inactive. Flashing stock & other alternative ROMs did not solve the problem.
So I returned my N4 to my seller. He accepted the warranty issue and replaced during the reparation:
1. Screen. So I have a brand new screen.
2. Mother board. So I have a new IMEI.
So this isue is rare and has hardware reasons