Even though you are using an External Runtime (mostly likely stored on an external drive), I have found through trial and error that M4 and V4 want to be installed to the initial runtime.  Therefore if Poser is installed say on your C: drive then you should install M4 and it's associated Morphs there as well.

  Once those Morphs are installed on the initial runtime, you'll be able to use the content on your external drives just fine.

  I know HD space is what you are trying to save and you wish to reduce the runtimes but it was the only thing that worked for me.

Therefore, your M4 install should be...
Install Poser (Done)
Install M4
Install M4++

Then try applying the morph packages you bought that are installed on the External Runtimes.

Good Luck!