Bluetooth Tethering on Galaxy S4
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.