Change MTP/PTP for a Galaxy S4 w/ a Broken Screen
If your phone is like mine, this option does not exist in any of the Settings menus.  First, connect your phone to your PC via the USB cable.  (If it is already attached, remove and reattach).  Watch your Notifications bar at the top of your screen and you will see a USB symbol appear.  Pull down the Notifications and viola, there is the option to change it back to MTP.