Interesting..
So, take this with full grain of salt because it's probably not the FX8 and something with my amp or my cables...
BUT.. I also ran into hum issues using 4cm, but only when I also connected a relay to my amp's footswitch input. I tested the relay and it worked fine, but there was a definite hum increase on both channels when connected. Enough to be annoying, but not unuseable.
But wait.. there's more!!!
I made my own 4CM cable snake containing 2 Humbuster cables and one regular, plus one with TRS on both ends for the relay channel switch. I used right-angle 1/4"connectors on both ends, for better fit in my pedalboard case, and so they'd hang straight down off the amp at the other end). Because the jacks are close enough together, it is easy to rotate/position the 90 degree plug ends such that the outsides of the connectors can touch. That shouldn't matter in the least, but I found that I'd get a MASSIVE buzzing sound as soon as the relay and out 1 plug ends contacted each other. WAY more buzz, unuseable. I double checked my cable soldering and it was all good. WTF?
So I was looking at either using straight 1/4"plugs to prevent contact, wrapping them in tape or some other material to isolate them... OR simply not use relay switching at all. Truth it, my amp has a great clean channel and I can get excellent crunch and lead tones for my purposes using drives on the FX8. So I'm probably just going to skip amp channel switching altogether.
The stock footswitch the amp came with has LEDs. I don't think there's a battery in it. I need to open it and check for sure, but if not there must be some low voltage on the wire just to light up the LEDs. Maybe this voltage is creating buzz with the FX8????