How to enable USB mass storage mode on Google Nexus 4? A big NO. Just like other Google Nexus devices, Nexus 4 does not support USB mass storage mode as well. You have Media device (MTP) and Camera (PTP) protocols only for USB connection. Or you can use Wi-Fi connection to transfer files between Nexus 4 and PC. Use free Android application like AirDroid.
Yes, I know some must use USB mass storage mode. But you are running out of luck with Google Nexus devices. You will have to change to other smartphone like Samsung Galaxy S3 or Galaxy Note 2 that support USB mass storage mode. Or you can use AirDroid as alternative to transfer files (of course, no USB mass storage here) with your favourite web browser on Windows, Mac or Linux computer.
Remember… Google Nexus devices do not support USB mass storage mode.