From Adobe’s Support Forum:
“To open an InDesign CS5 document in InDesign CS4, do two things. 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: Visit Product updatespage on the Adobe website, and choose to the InDesign from the product list.
Opening a document in a previous version of InDesign is also referred to as “saving down.”
Note: Content created using features specific to InDesign CS5 may be modified or omitted when you open the document in InDesign CS4.
You can open the .idml file in InDesign CS4 to convert it to an untitled InDesign document.
Note: To open the InDesign CS5 document in InDesign CS3, open the exported IDML file in InDesign CS4, save it and export it to InDesign CS3 Interchange (INX), and then open the exported INX file in InDesign CS3. Make sure that all InDesign versions are up to date.”
NOTE: In order to open IDML files from newer Indesign files with older InDesign Programs with correct type, you’ll need to make sure all Type is converted to Outlines. Then you when it is open in the older version, You can save it in that version.