Google has edited the online help manual for the Nexus 4. Prior to editing the online help manual, Google did have a section in the manual that mentioned that there may be the need for an adapter to connect some peripheral devices via USB. Now the Nexus 4 online manual has been edited to say the following on Google's site
You can connect a keyboard, mouse, or even a joystick or other input device to your phone via Bluetooth and use it just as you would with a PC. USB is not supported for connecting such devices.
Some of you may be asking why is a USB port or USB capability important in a cell phone? To some people it wouldn't be a big deal at all, to others it is a pretty big deal. The USB capability, allows people minimal amount of storage to add storage, if you will, using a USB OTG (On The Go) adapter (pictured below) and a USB flash drive. This would allow the people with the smaller gigabyte models (or any model of that matter) to save those large video files or large files that they can't store on 8 or 16 GB and basically allow everyone to have unlimited storage using flash drives.
There is another issue when dealing with connecting everything via Bluetooth the large battery drain. Not allowing USB connectivity also means that everything will be connected wirelessly through Bluetooth and the Google Nexus 4 online help manual even carries this warning "Important: Continuous use of a mouse via Bluetooth will cause a significant additional drain on your battery." People wanting to use USB OTG to help save battery life may not be able to in the future, at least not on the Nexus 4.
It is unclear at this time if the Nexus 4 will get any updates in the future to allow USB OTG connectivity, but for right now, the manual says it all, USB not supported here. We'll keep you updated if this changes.