Hi - the same 2 major tools for aircraft creation FSDS2 & Gmax are compatible with FS2004.
More info here
http://www.simviation.com/fsdesign_aircraft1.htm& on another note a press release received today:
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Herve Devred and I let us have the pleasure to announce the release of Aircraft Designer 2002 v1.2 FREEWARE.
This completely functional version is intended for FS2002 / CFS2 and probably FS2004 and it has been uploaded on your site from a short time. Name of the file actad2k2.zip.
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So another option on the immediate horizon.
Simplicity is the ultimate sophistication. – Leonardo da Vinci