Hi,
I am using the M13 with both my Roadster and my Nomad. I have the M13 setup so the first two "pedals" are pre-FX loop and the second pair of pedals are post-FX loop in the M13. You have to go into the setup mode to change the position of the M13 FX-loop in the chain. I run delays and reverbs post-FX loop and distortion/overdrive and chorus pre-FX loop. With the four cable method, the distortion/overdrives will feed the front of the preamp and delays and reverbs will enter the amp before the signal goes to the power amp.
Take the send from the FX loop on the M13 and plug it into guitar input on the amp. Take a cable from the Roadster's FX send and feed it into the FX-return on the M13. Take the output on the M13 and insert it into the FX-return on the Roadster. The fourth cable goes from your guitar into the input on the M13. Make sure you take the FX-loop off bypass on the Roadster and make the FX-loop active on the channel you want to use.
Look in the manual for the M13. It has a lot of different setup possibilities, including a picture of the four cable method.
HTH