Indesign CS5 can open files from previous versions of Indesign, however previous versions of Indesign (CS4) cannot open files made by Indesign CS5.
The way to be able work on a file created in Indesign CS5 in a CS4 is to export the file as an IDML (InDesign Markup Language) file.
To do this go to File>Export
Once the Export Dialog box is selected make sure the file format is set to IDML (InDesign Markup Language)
You should not be able to open the file in Indesign CS4*
*Make sure you have run all Adobe Updates to insure compatability