I've had a quick look at an FS2k4 AIR file, and there doesn't look to be much of a difference to FS2k2 in terms of structure. FS2k4 itself does change some of the flight characteristics (e.g. drag), but I think this is more to do with values inserted, than with the file structure.

Where you will notice a difference is in the aircraft.cfg file, which has more entries for additional variables in FS2k4, where FS2k2 just ignores them. So perhaps this is a case of the new sim relying on the cfg file even more than FS2k2 does.

Some planes are 04 compatible, as they were designed for 02 originally, but 'converted'. There are lots of those around, so compatibility shouldn't be an issue. Where there is a plane specifically designed for FS2k4 (and the Posky 757 is a good example) where it states that it's for FS2k4 only, it can indeed be flown in FS2k2 (I do it with this plane), but might (or might not) need a tweak in the aircraft.cfg file (wing incidence) because of the drag issue mentioned above.

Hope this helps.