I don't believe your Spider IV Head has an effects loop send and return.  I checked the manual, and didn't see one.

The problem that I think your going to have will occur if you want to creat a loop with the JM4, and then change your "guitar sounds". 

Putting the JM4 in front of the amp, will loop a clean signal from your guitar, and pass that clean loop into the Spider.  Once you change and effect, the playing loop will be processed through the new effect.

I've done this with a Trace Elliot bass amp, which has an effects send and return.  I use my effects into the amp, which is then routed out the effects send, into the AUX input on the JM4, out the JM4 back into the effects return, and out the speakers.  Doing this allows me to record the effect as I playing, and loop that while I change to another effect, and then record again. 

If changing effects sounds are not a problem, then I'd personally use the AUX input on the JM4 to completely bypass any amp modeling from the JM4.

Don't know if that makes any sense.

Joe