Cant open GB3 file in GB4

Hi,
I can't open a Garageband 3 file in GB4. I get the updating to new song format dialog box and endless beachball spinning. The file opens fine in GB3 on my G5 work machine. Other files with the same software instrument tracks open fine in GB4 as do different GB3 files. Eventually GB4 crashes.

I have a new 3gig iMac with 4 gigs of Apple ram.

Moved the com.apple.garageband.cs and com.apple.garageband.plist files out of user/library folder. No change. What could be the problem?

Keith