I have updated the samsung galaxy s4 OS from Kitkat to Lollipop using system update. Now I want to hard reset. Does anyone know that after the hard reset Lollipop OS remains or it reverts to Kitkat?
Hard resetting simply removes all of your data and apps to make the OS like you installed it. If you wanted to revert to Kitkat you would need to flash the ROM.
So no, hard resetting does not revert to Kitkat.