so to run the RM4 into the RM100
you need to split the guitar signal ( two ways, once with a single Y cable that goes from last pre gain device to head guitar in and RM4 guitar in, and the other is using an axess BS2 buffer which has two isolated outputs that will feed both guitar inputs of RM100 and RM4.
next thing is to use a looper as an Axess GRX4.
the best is to use the loop 4 which is isolated from the others,
you plug this way :
RM100 SEND -> LOOP IN
RM4 OUT -> LOOP RETURN
LOOP OUT -> RM100 RETURN
this way LOOP ON = RM4 and LOOP OFF = RM100
that's all