Yes I now but the solution with revert.zip like in the first post would be much easy So if I understand with flash system.img it's like install an OTA : I loose root but no program and data with a "classic" installation from the play store....except the programs and data that may have been manually installed in system partition, for instance root, busybox, cerberus, etc... But them can be reinstalled too.
I will try to find system.img for my N4 but I'm sure that I can find it on XDA
Thank you for your help you and other members.
Usually I try alone and test many things on my pad (TF101) ... with big fear sometimes [emoji4] (no boot, loose data's, etc ...) But as I need my phone to work I want to be sure !!![emoji4]
Long life to XDA !