JoJo
The mesh files should work OK in FS2004, however the terrain resolution may appear to be not as good in FS 2004, the reason being the reduced resolution set by Microsoft in FS 2004. If you have installed the patch to take FS 2004 from Ver 9 to Ver 9.1 the resolution issue will be resolved. I use the Terrain mesh software "FS Global 2005" although it may appear expensive, it covers most of the planet except the polar regions, so you do not have to bother about installing terrain mesh again.