What woul happen if i got a GNexus and used a Nexus 4 ROM? Would it brick?
ClockworkMod will read the headers before attempting to flash and throw out an error.
The real problem with flashing just anything without a confirmation is basically that it can contain some additional things. Like a baseband upgrade. If you were to apply a ROM with one that wasn't for your phone, you're totally going to brick it to the point where you can't make calls or use the internet anymore (or you might lose 3G/LTE). It's a firmware patch and there's no going back once you apply it.
There are instances where multiple phones pretty much are the same device though. In my case, I have a Huawei Fusion 2 which has the same chip as some of their nicer devices. I'm using a ROM on this $60-80 GoPhone from a device that is like $300.
But yeah, the Galaxy Nexus is completely different to the Nexus 4. It was made by Samsung vs LG to start with and they use completely different hardware, if not the chips, the displays themselves. Don't be afraid to install an unofficial ROM though, as it's all just Github code that anyone can compile themselves with the right tools. And a ton of people test it out to meet the same stable state of the officially supported builds.
Big no no
