There is only one way: open your .indt file in CS5.5, export to IDML, open that in InDesign CS4.
And live with the consequences ;-)
There are a lot of features in InDesign CS5.5, that will not travel over to CS4. Most important maybe paragraphs that span columns; be aware of recomposition of text, etc.pp.…
And, of course, different page sizes in one document…
Prepair yourself for a lot of trouble.
sounds like I should upgrade doesn't it.