First, in InDesign CS5, export the document to the InDesign Markup Language (IDML). (The IDML format replaces the Interchange INX format used for saving backwards in previous versions.) Second, make sure that the computer running InDesign CS4 is updated with the appropriate plug-ins so that it can open the exported IDML file. The person running InDesign CS4 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.