If you have a Nexus 4 and and a mac computer and want to transfer files (like a mass storage device) than a Mac users need to go to
Android.com/filetransfer/ and install the filetransfer app onto their computer. Also on your nexus phone go to settings by swiping the top black bar (with the battery and wifi icons) downwards and select the little whit icon with the person surrounded by square dots, then choose "Storage", inside storage, select the three dots at the top right and make sure you have "MTP" ticked.
Once I transfered music to the music folder I fiddled around with my google play player and it showed up.