I just came across this thread, trying to get my SG4 I9505 to switch to USB Mass Storage mode, instead of the useless MTP mode.
The app called "Mass Storage Mode Note3*ROOT" from "Sanjo" works for me.
Its a very simple app, that does only and exactly that - switches between UMS and MTP.
UMS-mode sticks around even after disconnecting and reconnecting USB-cable, but after phone-reboot I need to run the app and click "UMS" again.
Please note: The app requires ROOT.
Please note: My SG4 I9505 is running a wanam lite 4.4.2 rom (which is quite close to stock, but much nicer optimized and perfectly stable).
I hope this helps someone out there (this is also my first post here