Yes, I'm using the 4CM. This puts the pre-amp of my amplifier in the FX-loop of the GT (and bypasses the pre-amp of the GT). But in trying this I discovererd that the 4CM only uses one channel of my amp. So channel-switching isn't possible anymore...
I use the GT only for wah, modulation and delay FX. For drive I use my amp. And as I said before, it would spare me some space on my pedalboard if the chanels could be switched by de CTL-pedal of the GT. But I've searched on the internet and in the manual, and I don't think it's possible.