Do the scenery add-ons from FS 2002 work in 2004? Also, does FS 2004 have a building editor like 2002 pro did? If not, would the one from 2002 work in 2004?
So far the vast majority of sceneries from FS2002 seem to work OK. The only problems I've found is that some of the alpha channels are different so instead of a see through you might get a pink window pane or pink around the tire of a car or truck. Another problem is that some of the coast lines and water ways are placed differently. If the scenery is for an area like that you may have a float plane dock that's located inland or a FBO building floating out in the water.
I haven't run into anything catastrophic yet. No crashed sim or anything of that sort. You just have to try the scenery out and if it doesn't work to your satisfaction you can delete it. If you are going to use scenery designed for FS2002 I would suggest placing every scenery in it's own folder inside the "Addon Scenery" folder. That makes it easier to add and remove scenery. Don't divide up the files and place them in the "Scenery" and "Textures" folders inside the "Addon Scenery" folder or you might have trouble finding one that's giving you problems.