The recipient of the device, Jeff Williams, who snapped it up on US used goods site Craigslist a couple of weeks ago, prompty checked the software version and found Android 4.3 Jelly Bean listed in the phone's settings app.
Williams posted a screenshot on an Android developers site, who deemed it legitimate and helped him extract the software and make it available for others to install on their handsets.
However, those who have installed the update on their Nexus 4 handsets, including the good folks at Android Central who posted the video below, aren't reporting too many changes from Android 4.2 Jelly Bean.
The build brings OpenGL Enhanced 3.0 graphics (the new mobile standard), which should benefit gaming, more efficient, low-powered Bluetooth LE connectivity and improved touch sensitivity for the Nexus 4.
The new camera interface that previously appeared on a Google Edition of the Samsung Galaxy S4 handset also appears within this new leaked built. The arced options interface allows users to toggle shooting settings quickly and easily without entering a separate menu.
Beyond that, most other changes detailed in this particular build of Android 4.3 appear to be back-end updates or tools for developers, with the average smartphone user unlikely to see much difference.
There was also no sign of the Open Mic always-on voice command feature which has been showcased in leaked videos previewing the forthcoming Moto X phone. It is possible that it hadn't been added to this particular build of Android 4.3, and will be present in the final version. However, it may turn out to be a Moto X exclusive feature.
The update is expected to be officially unveiled at a Google event next week, with members of the media invited to breakfast with Android chief Sundar Pichai on July 24.