FORSIM IV was developed as an aid to simulating certain aspects of the Back-Up Interceptor Control (BUIC) system, and is the first general simulation pseudo-language developed for the IBM 7030 computer. It represents an innovation in simulation language technique since it is constructed not as a language, but as a subroutine package. It can be adapted to any computer capable of compiling programs written in FORTRAN IV language. Constructed in FORTRAN IV, FORSIM IV is conceptually related to Control and Simulation Language (CSL); however, it provides commands and services not available in CSL, while its subroutine structure provides for the easy expansion of the command set, as well as virtual machine independence. A model FORSIM IV program is included in the document.