CS4 is no longer sold through regular distribution channels. The only place you'll find it is the aftermarket, and be very careful. Education licenses are not transferable, and anyone who used CS4 as part of an upgrade chain must transfer the license to all versions in the chain together (which is not likely to happen), plus there are a LOT of counterfeiters.
Peter’s right but perhaps you could expand on the compatibility issue.
Thank you for your repsonses
We are providing translation of indesign files and our software will only accept the inx format. So really all we want is a converter to change idml to inx, that's all we are using it for.