First of all, HUGE thanks to jjdoctor and Alfons_Hetler for their work. I am running KK 4.4.4 on a rooted S4 SGH-M919 (T-Mobile USA), and had no luck with the earlier solutions based on the USB Mass Storage Enabler app (
http://xdaforums.com/showthread.php?t=2297888). The following has enabled UMS without a hitch:
1. Download & install SManager script editor from Google Play Store
2. Create folder \KitKatUMS somewhere on the SD card
3. Place UMS & MTP script files from Alfons_Hetler's post (#27) into this new folder
4. Open SManager and open the UMS file
5. Select "Su" (green skull and crossbones icon), "Boot" (cogwheel icon), and "Bg" (& icon). (Note:Selecting "Boot" will ensure that the script enabling UMS mode is automatically run each time the phone starts up )
6. Select "Save" and "Run"
Note: If the phone is still in MTP mode, a one-time selection of Mass Storage Mode may be required within the USB Mass Storage Enabler app. I already had this app on my phone, before coming to this thread, and don't know if it's a prerequisite for having everything work