We made it easy:
https://www.antennasdirect.com/store...tenna-Kit.html
You can either use the integrated diplexer in the module with the amp set to "combine" or you can use the amp in the "separate" mode and cap off the UHF input of the module with a terminator and feed the C4 into the amp's UHF port.
I'd recommend extending your existing mount by 2' and attaching the VHF kit to the top of the extension.
I'd also strongly recommend that, while you're up there, you install a strain relief on the coax from the C4's combiner assembly to prevent breakage of the circuit board inside the combiner. In the event it should fail, contact customer service for a free replacement. The new ones won't break very easily.