The black extension tube that comes standard with the C4 is 8 & 3//8" long. The clear portion of the intake on the filter out of the box goes to about 5 inches below the water level so the total means you can go to about 13 inches below the water level fro pickup.
According to the Hagen website, the part numbers are in fact different for the intake tube and the extension tube:
A20283-Fluval C4 Intake Tube - Intake Tube is suitable for Fluval C4 Power Filter
A20285-Fluval C4 Extension Tube - Extension Tube is suitable for Fluval C4 Power Filter