I recently wanted to update my Nexus 4 to Android 5.0 but I wasn't able to because I have apparently changed something in my build.prop file, so OTA updating doesn't work. If I flash a clean Android 4.4.4 system.img will it fix this problem and will it wipe any important files? Thanks in advance. :D
You can just download the lastest system image for your device and flash it right to lollipop if is avaible to your device of course.
I think Nexus devices have their system image avaible directly from google so google for it download and follow their steps and done your nexus is now lollipop.
If not find the system image in google site try xda-developers forum and you find it there it all the steps + the image.
When you flash a new system image you just replace the older ROM you can flash any system image to any phone if of course your device supports it.
If happen to you flash an wrong ROM for your device you will only soft brick it and you need to reflash again with an supported ROM.
This erase all your data so make an backup before.
It is strongly recommended to do an factory reset and wipe cache data after every change/upgrade of ROMs to avoid all kind of problems.