Yes, the file format is forward-compatible. Not just the file format, but more important, the object definition for each object. OM 5.x docs open in 4.x with no problem at all. Which really means, you open a 5.x doc, but you can only perform 4.x functions on the objects. You can even open 5.x stencils in 4.x.
I have not yet found "5.x-only content", I am not saying it doesn't exist, just that it is rare, and when you find such, you can fix it. AFAIK, there is either 4.x-functions or 5.x-functions on objects which are not version-specific, within limits and reaching the limits is rare.
The only "5.x only content" I have seen was very badly written (in 5.x) templates; once I rewrote the objects in simpler form, and re-saved the doc as a simpler 5.x template, it loaded perfectly into 4.x. Complex 5.x objects in templates cannot be rendered in 4.x. What I am saying is, if you have a need to frequently open 5.x docs in 4.x, keep them clean and simple, and keep the 4.x limitations (not really limitations) in mind.
Last edited by DerekAsirvadem; 2010-05-14 at 01:11 AM..