Are FS2004 aircraft compatible with FSX?
Yes, you should be able to fly FS2004 aircraft in FSX. However, some add-on aircraft may cause problems with certain gauges, modules or other additional software. Some payware developers released or will release updates in order to make their products compatible with FSX. Freeware developers may do the same if nessecery, but since it is their hobby and not their job, some might choose to move on with the development of new aircraft for FSX only and keep their current products for FS2004 only.