dyld: Library not loaded: /usr/local/lib/libmpfr.4.dylib
@Vinc17 is right in his comment to @neosarchizo's proposed solutiion, symlinks can be problematic if referencing a different version, HOWEVER, in my case, running the following command:
locate libmpfr.4.dylib
Showed that I did have the right library in a different directory:
/opt/local/lib/libmpfr.4.dylib
So for me, I felt it was perfectly safe to use the symlink approach, ie:
sudo ln -s /opt/local/lib/libmpfr.4.dylib /usr/local/lib/libmpfr.4.dylib
Probably a hack, but problem solved.