PS I just upgraded to 4.03 from a 4.0 beta. I'm a life time member.
From my experience this is the culprit and you shouldn't have any more problems, Was this your first sync with MM4? I had to erase all of my devices and start over once I upgraded to MM4 (iPhone, iPod Classic, and two Sansa Clips). Once I erased and resynced all has been well with all my devices.
Seems as though MM4 couldn't "read" what was on each device and therefore didn't know what to sync.