What I do not understand is why anyone would want to try to use FSX aircraft in FS2004.Other than the default planes which come with FSX, there are precious few true FSX aircraft available.A large majority of the so called FSX aircraft are FS2004 planes which have been tweaked so they will work in FSX - very few have been created from scratch following the FSX guidelines completely.Other than the default FSX aircraft what might there be to gain?Aircraft of almost every type has already been modeled for FS2004 and there are hundreds of repaints in almost every conceivable livery available also.To answer the OP question though, I am reasonably sure you cannot use FSX aircraft in FS2004.

Maybe they want the planes of FSX with the performance/looks of FS9?