Make sure that the computer running InDesign CS4 is updated with the appropriate plug-ins so that it can open the IDML file.

You can obtain the compatibility plug-ins by choosing Help > Updates and following the prompts. Plug-ins can also be installed from the Adobe website: Choose Support > Updates in the Adobe website, and go to the InDesign Downloads page.

See Save backwards to the previous InDesign version at help.adobe.com