TS-UGOS 4

 (codenamed Sydney) is a graphical operating system developed and released by Theorysonic. It was released on May 11, 1991.

History
The development of TS-UGOS 4 began on April 25, 1989, thirteen days after the release of TS-UGOS 3.

New features

 * RealityX API for game programming and multimedia, replaces the old TSGI API for improved features and technology. Limited 3D support.
 * Rounded taskbar.
 * Limited support for 24-bit RGB, with need to install additional drivers.