What plugins are listed as missing?
Waht makes you sure it is a CS4 file? If you don't have CS5 or later, the sytem will not identify later files as anything other than the ast version you installed. Jongware wrote a script to help you identify the version of ID files: Adobe Forums: [Ann] Identify Your InDesign File
If the missing plugin error is jusat for the InMath plugin, there's a pretty good chance that the file will open fine, and you can remove the plugin information (and the error message) by exporting to .inx or .idml. See Remove minor corruption by exporting
the file only looks like a CS4 file on your Mac because you don't have the latest version installed.
You can determine unequivically the version of a file by using the script that Jongware posted here: Adobe Forums: [Ann] Identify Your InDesign File