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?
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.