There is a known bug in ACID Pro 4.0 build 215 that crashes the program when you open a MIDI files created from ACID Pro 3.0. This issue has been addressed in updated versions of ACID Pro 4.0a or later and is available for download from Sony at the link below. The only work around in the previous build of ACID 4.0 is to go back into ACID Pro 3.0, get into the Track Properties for that MIDI file, and then Use the Save feature located there to save out your changes.
www.sonymediasoftware.com