Broken Glass...
had it in my pocket, took it out and have a crack by the cam lens, like a spider web with a few hairline cracks across...
just like this one.... http://phandroid.com/wp-content/uploads/2013/04/Nexus-4-cracked-featured.jpg
been googling and i see loads have the same issue and the same crack by the lens, the pick above could be my phone, its almost identical
forked out £50 for a new back direct from LGnot happy as it was not dropped
http://www.spareslg.com/tampa-de-bateria-lg-optimus-nexus-4-e960-acq86231201.html
Certainly from doing a search there it does seem that the back can crack fairly easily which isn't great to hear, however other folks have said that they have dropped theirs and it has survived ok. I was going to order a Nexus 4 and it is a little bit off putting to read this although due to the nature of my job I was going to order a sturdy case anyhow, something like what Psycho Sonny had linked to or maybe a Seidio Active case and hope that would keep it well protected.
Hope the replacement goes well enough for you, at £50 a go you wouldn't want it happening too many times.