Old versions of of Digital Performer can't open DP4 files. However, you can convert a DP4 project into an older version (DP3.11 all the way back to DP 1.6).

1. From the FILE menu, choose Save As (or Save a Copy As)
2. In the dialog box, click the pop-up next to FORMAT and change it to the desired DP version.
3. Name it and selected the destination on your computer to save it to.
4. Click SAVE.