To use BT-4 with BIAS FX 2 Mobile, you'll need to set up the buttons on BT-4 to MIDI Control Change or MIDI Program Change.
1. Open the BT-4 Pedal Editor and set the buttons to "MIDI Mode -> Control Change":
2. Make sure the Channel is set to 1 with the number 0 - 3 (or 1 - 4) to four of the buttons on BT-4.
3. Now you can assign CC message 00 to 03 to the MIDI assignment in BIAS FX 2 Mobile.