Here are the controls for Spider IV amps (75,120,150, HD150) when using the FBV 3 foot controller.
FS1: Boost on/off
FS2: FX1 (Gain, Auto, Pitch)
FS3: FX2 (Chorus, Phaser, Tremolo)
FS4: FX3 (Delay, Tape Echo, Sweep Echo)
FS5: Reverb on/off
Function: turns on looper mode.
Tap button: will start and stop the recording, overdubs, playback within looper mode.
When using the FBV3 with your Spider IV, make sure you have the latest flash memory (v1.02) installed for your FBV controller.
You'll find the Line 6 Updater located here: http://line6.com/software/
Download and install Line 6 Updater, then connect your controller to your Mac or PC with a standard USB cable. Line 6 Updater will ask you to login, and then run the latest update for the controller.