One thing Adobe has not always done well is maintaining backward compatibility in their applications. This may not be such a big deal if you work in an isolated environment or only associate with those who meticulously upgrade their software to the latest available versions. However, many real-world projects get passed around between designers, clients, and various vendors—all of whom may not have the same version.
Adobe Illustrator was notorious for this, but also offered the graceful solution of simply saving “down” to a lower version. InDesign, however, is not quite so well behaved.
There is hope. InDesign can export to what is appropriately called InDesign Markup Language or IDML. This IDML can then, in turn, be opened in CS4.
If you are doing this to send to someone else, make sure you provide a PDF so they can have something to check against as there is always the chance that something could be off just a bit.