I actually have an M4 and a Mod50. I am using the Mod50 as power for the M4, and I sounds great. The M4 actually feels like it's a little cleaner, and has more oomph than the Mod50 input, but that could all be in my mind.
M4 all the way for me. Get that a great head....and a switcher....use the preamp from both the head and the M4 into an amazing power section like a JCM800.
I like the Mod50...but ultimately my island power section is still the Marshall Silver Jubilee 2555. Hence why I built a modernized clone of one!