.indb files cannot be saved backward. You must export the individual .indd files to .idml, open and convert those in CS4 (and don't expect an exact match), then add the new files to a new CS4 book.
Thanks for your quick response.
this means a lot of work
Or an upgrade.
Yes Bob, that was my first reaction too.
Not exactly. There are scripts around doing the tedious task.
E.g. see the following by Peter Kahrel which brings you one step further:
For security reasons do not overwrite your InDesign documents.