C4 and EQ2 work the same as each other. Aftershock and Mercury work the same as each other. For the former two, you set a minimum and maximum percentage in the external control section of the editor. For the latter two, you specify a start and stop position for the knob(s) you map the expression control to.
For C4, connect the Hub to the pedal via the TRRS cable.
Plug a 1/4” cable into the Hub’s expression input.
In the C4 editor, switch on the “external control section” (near the bottom left of the screen).
Select “Hub/external Midi” as the source on a slot, choose a destination parameter. (Obviously this needs to be a parameter that is in use in your current patch - setting it as “voice 4 pan” would be pointless if voice 4 weren’t switched on.)
Set a minimum and maximum percentage. 0-100 gives full range, 100-0 reverses the sweep direction.
If you want to to have the parameter’s current knob position to be your starting point then you need to divide it’s current value by 256 then multiply by 100 to give the percentage value you’ll need to use as your first value. Enter whatever value you want for the second value to give the desired result (higher if you want the knob value to increase when you press the expression pedal, lower if you want the value to decrease when you press the pedal).
For the Mercury it’s a similar process but instead you only have a limited choice of the the four hardware knobs to map to expression. Then use the dedicated dials to specify the minimum and maximum knob position of each parameter you wish to control.