You can’t really convert it to a pure ID.. you can import it of course and then create a new pdf to the specs.
I haven’t used CS4 so I’m not sure if there is a plug-in or new option but of course it would matter if the old pdf was created correctly rather than being totally rasterized or exported from Photoshop as a Photoshop PDF. You would have to be able to extract text and images.
On the other other hand, you can just export text and images from the pdf in Acrobat and then rebuild the layout in InDesign.