They control the 4 parameters adjacent/after the parameter the Value knob is controlling. If you're controlling a Graphic EQ you can tweak 5 values without having to select a new value to tweak: If you have 125hz selected, the value knob controls 125hz, A controls 250hz, B controls 500 hz, C controls 1K, D controls 2K.
If you're on the main page of an Amp block you can control Drive, Bass, Mid, Treble and Presence from the 5 knobs. Push the > button and your controlling Bass, Mid, Treble, Presence and Depth.
Having 5 knobs to adjust a group of parameters greatly enhances the editing experience for me - I now use both AxeEdit and the front panel, but the Value and 4 Control knobs are now my favorite way of adjusting parameters.