A large series of add-on products were produced for FS4 between 1989 and 1993. First from Microsoft & the Bruce Artwick Organization (BAO) came the Aircraft and Scenery Designer (ASD) integration module. This allowed FS4 users to quite easily build, on the fly from directly within the program, custom scenery units known as SC1 files which could be used within FS4 and traded with other users (this activity was quite popular in the FS Forum on CompuServe). Also, ASD provided the addition of the Aircraft Designer Module. Again, from directly within the program the user could select one of two basic type aircraft frames (prop or jet) and proceed to parameter customizations ranging over 4 pages of flight envelope details and visual aspects. Finally, ASD provided additional aircraft including a B747 with a custom dash/cockpit (which required running in 640 × 350 resolution).
Source: Wikipedia, History of Microsoft Flight Simulator, available under the CC-BY-SA License.