RM4 switchable series effects loop?

Does the RM4 have a midi-switchable series effects loop?

Don't believe so. The parallel loop has a blend (or level) control on the front panel.

I'm planning on using the RM4 into the effects return of my RM50B since I feel restricted by 2 modules. I'm trying to figure out an easy (and free!) way to midi switch between the 2 modules in the RM50B and the 4 in the RM4. Ideas? Worst case, I could get another Axess GRX4 to do the job, but free would be better.

A midi controllable a/b box would be your best bet. I looked into this option for when i was thinking about keeping my RM4 after I got my RM100.
Plugging guitar directly into the A/B box, then A into RM100 and B into RM4 & running the RM4 into the series return of the RM100 would have then given me 7 available channels all midi switchable as both the RM4 and RM100 have midi in out, i don't believe that the RM50 is midi switchable though? which may make things more complicated for you.
Anyway here is a link to a midi switchable a/b box that would do the trick

http://www.soundsculpture.com/products/pt_abcadabra.htm

Goodluck I hope it helps.

I wonder if the parallel blend is MIDI Controllable via CC commands from an expression pedal or something. :?

Midi Thru of Amp1 into Midi In of Amp2 I guess.
But is it also possible to change the CC numbers of 1 amp?