We don’t know why, but for the longest time, Google did not distribute the Nexus 4 factory image. They took it down from their site along with the binaries and never spoke about it. While we are very curious about what happened, we can at least say that the factory image for 4.2.1 is now back up on their site. The binaries are still gone, but at least we have something. If you ever encounter a software problem and need to return to stock, you can now download the factory image from an official source. Of course, we recommend you download it now and keep it around in case it ever goes missing again. Hit the source link to download it.