Multi-user support in Android 4.2 is really cool, but it's only available on tablets. Courtesy of the MoDaCo Toolkit however, you can also enable it on your Nexus 4 (and quite possibly other 4.2 based phones).
Once you've done that, load up the MoDaCo Toolkit and click the 'Google Nexus (all)' section. Click 'Show users on power menu', reboot your device and you're done. Easy eh? :)
Now that multi user support is enabled, you need to know how to add and switch users. You can add users via the 'Users' menu that now appears in 'Settings'.
You can switch users by pressing and holding the power button and selecting the name of the user you want to switch to. Simple! :)
Now, I know this works on the Nexus 4, but if you try it on another 4.2 device, please do let me know how you get on! :)