This is my story, I'm in the ZV4 and I modified my build.pro and now I'm in a bootloop. I tried to return to stock using the ZV4 tot file checking UPGRADE DL (I hadn't seen this tutorial) and it failed saying "device model is different, check phone or DLL". After seeing this tutorial I tried checking BOARD DL but it says the same, and I tried with Generic LG G3 DLL but it gives me another error: "Cross DLL [LS990] to [LS990]". I don't have TWRP installed so I can't use it (I know I should have installed it and made a backup). I have stock recovery and ADB says device offline.
Please, help me with this I don't know what else to do.
I was able to solve the problem. I went to the PC where all started, and there I was able to use ADB with root permissions. I erase the build.pro file and after that I was able to return to stock using lg flash tool.
So the problem is when you have a corrupted build.pro file, because the lg flash tool tries to read the model from there, but if you don't have a build.pro it doesn't check the model and bypasses this check.