FB4 are no longer made.
The FB4 gives any Line 6 Digital Guitar Amplifier System simple Channel Select and Tap Tempo control. It's pretty darn nifty looking - with a shiny red anodized metal body. And using it is pretty darned simple.
Step on one of its switches to choose one of the four Channels of your Digital Guitar System.
You can also access the Tap Tempo function by tapping on the currently lit channel's selector switch. Tap two or more times to set the tap tempo.
The FB4 includes a locking cable, and connects to the Digital Guitar Amplifier System via the jacks on the side of the FB4, and the back of your Line 6 amp. The cable allows the two to communicate, and also supplies the FB4 with power; so there's no separate power cord or wall wart needed for the FB4.