Do you have M4 installed to the main Poser runtime or to an external runtime?

Are all the morph packs installed to the same runtime?  The Muscle morphs must be installed to the same runtime as the regular M4 morphs.   

In whatever runtime you have M4 installed to, you should find a folder labelled !DAZ inside the library folder.  Open it up and run the .bat files for M4 you find there.

A last thing to try if you are using an external runtime is to copy the PoserPro.exe to the external runtime and rename it Poser.exe.  For some reason in the early days of V4/M4, there were issues because the Pro versions of Poser used PoserPro.exe and the other versions of Poser used Poser.exe.