Google has edited the online help manual for the Nexus 4, removing references to USB support for mice and keyboards. Previously mentioning USB and that you might need an adapter to connect these peripherals, now it simply states
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
Taken at face value this isn't that big of a deal, but any effect it may have on USB OTG (On The Go) could get a bit more worrisome. Many were counting on support for USB flash drives to supplement the 8 or 16GB of internal storage, and if the Nexus 4 can't properly power USB OTG devices that can't happen.
We're not going to say USB OTG support for the Nexus 4 is dead just yet. It's a complicated affair, needing the correct hardware and software to properly function. If this is just a software issue and is a temporary "adjustment" there is no worries, but if the hardware can't properly support USB gadgets or provide the right voltage, we're likely to be out of luck. We've reached out to Google, and hopefully someone gets back to us Monday. We'll let you know whatever we find out.
Source: Nexus 4 online help