[Jean Benoit]

No, CS4 Plays all oldest format, But must convert the, Is that things can change certainne, Finally, yet I did not have to worry.

PAR against the opposite can be done.

Wholesale:
Of CS3 to CS4 = Ok
Of CS4 to CS3 = Not possible (unless you save your work for vertions CS3, Why save your document as usual and after the application of the name of the document it will ask you the necessary compatibility.)