I would need more information. You can also search GEANT4 Hypernews for a possible answer. I have definitely used G4 (version 4) in multi-threaded mode on one machine, but not MPI. For me, doing it required following the simpler (non-MPI) examples, but my guess is that you have already done this.
However, I will need to do this soon for an up-coming project. Will keep you posted.