Sony have revealed to Eurogamer that PlayStation MOVE is limited to four controllers on any one PlayStation 3 system.
This news means that any users with two MOVE controllers and two sub controllers will not be able to use any further motion controllers on the same system at the same time.
Having scoured the internet for an answer regarding this we have found out that Sony clarified the matter at GDC, saying,
Four PlayStation Move controllers can connect to a PS3 at one time (or two PlayStation Move Controllers and 2 PlayStation Move sub-controllers)
Whether this changes in the future with the release of the MOVE bundled PS3s remains to be seen, however for now at least it seems that if you want to play with four players at once, then you will be limited to one MOVE controller each.