No official announcement on this has been made so far
LG and Google appear to have made a series of design changes to the Nexus 4 smartphone in the latest units that have started shipping to users recently.According to the latest news on the matter, the new devices no longer feature the glittery back cover that was present on the previous units.
A recent post on Auto-oMobile explains that the newest unit they received no longer delivers the effect, regardless of the angle or light conditions.
For the time being, no official info on the matter has been provided by Google and LG, though they might offer a formal statement on this should the change affect all future units.
Recent reports suggested that only a small number of Nexus 4 devices have been produced and shipped so far, and it remains to be seen whether this was merely a production error or not.