Microsoft Flight Simulator Version 4 (FS4) was released for MS-DOS computers in late 1989. It brought several improvements over FS3. These included improved aircraft models, as well as an upgraded model of the Cessna Skylane, programmable dynamic scenery (non-interactive air and ground traffic on and near airports moving along static prerecorded paths). The basic version of FS4 was available for Macintosh computers in 1991. Like FS3, this version included an upgraded converter for the old subLOGIC Scenery Disks.
A large series of add-on products were produced for FS4 between 1989 and 1993. Some add-ons allowed users to create custom scenery, aircraft, and scripted adventures. Others provided upgraded graphics and audio.
FS 4 was also compatible with a large number of scenery add-ons. Scenery files produced by subLOGIC could be used with FS4, including subLOGIC's final massive USA East and West scenery collections.
With its many options and add-ons, yet still relatively tight "in program" integration and overall ease of use, the FS4 suite of programs presented a type of VR Toolkit for users with a flight simulator slant. While complex in some aspects, FS4 environment building options (including scenery and aircraft design) would provide an unsurpassed access to these activities for average users; an option, which in later versions of FS, was much less available and increasing complex.
Source: Wikipedia, "History of Microsoft Flight Simulator", available under the CC-BY-SA License.