Bluetooth Tethering on Galaxy S4 I have a galaxy s4 and a tethering data plan and I can tether via wifi (Settings| Connections tab| More Networks| Tethering and portable hotspot| Mobile hotspot). I don't see any options for bluetooth tethering. Do I need a special app for bluetooth tethering?

 
Before and after turning on Mobile Hotspot (click images for larger variant)
Bluetooth tethering is a native function supported by Android. You do not need third party apps.

To tether your internet via bluetooth on your Galaxy S4:
Go to home Settings ► Connections ► More networks ► Tethering and portable hotspot ► and check Bluetooth tethering.

As explained in this answer, you can tether two mobile devices easily by pairing the devices and enabling data on the first device.