Does anyone know of a way to use FS98 adventures in FS2004?  If there is no converter program, does anyone know how to decode the .adv, .flt, and .stn files so I could recreate the situations in FS2004?  Thanks!

No guarantees whatsoever ... but, from FS2002, in the FS2002\flights\fs2kadv\ folder, I found the following :-

[MAIN]
Title=Flight Simulator 2000 Adventures
Description=If you have Adventures designed for Flight Simulator 2000 that you want to use in Flight Simulator 2002, put the files in the fs2002\flights\fs2kadv folder on your hard drive (where "fs2002" is the folder in which you installed Flight Simulator 2002). Flight Simulator 2000 Adventure files must be placed in the fs2kadv folder to work properly. The Adventures will then appear in the Flights list below.

Try it and see 😉