You were running Nema 23s before--About 200 ounces, judging by the size. You DEFINITELY don't need the 906s.
The G540 can't really USE anymore than 8A, so I would go with the 7.3A PSU. That should be sufficient for all four of your axes. The 381 motors run better with G540 than 387s. This is what I recommend:
http://crevicereamer.com/Page__57.html Just add an extra 381 for your fourth axis.
You should be able to run the 381s geared 1:1. That would give you 250-300 IPM rapids--Or up to 7600 mm/min.