You don't just need a Series loop to run 4cm with the GT-8.
You need one to improve the tone of just about any MFX cause most of em sound better connected to the FX Return loop of your amp. That way you don't have the MFX's preamp running into the Mesa's preamp - result is usually a muddy tone.
If you have a parrallel loop you nearly always get an audible out of phase double tone. With a Series loop the two tones are mixed seemlessly. Basically, if you have a parrallel loop you're stuck with running any MFX thru the front amp input. It won't sound very good and that's one of the main reasons people slag off MFX's like the GT8, Tonelab and the Pod. They're usually right IMHO but it's largely because they've not set up the connections to optimise tone. Kinda like setting all your amps EQ pots on zero and then complaining you've got no bass or treble