  This paper describes the design and implementation of a facility simulator
for the 4 metre Multi-Object Spectroscopic Telescope (4MOST) project, a new
survey instrument proposed for the ESO VISTA telescope. The 4MOST Facility
Simulator (4FS) has several roles, firstly to optimise the design of the
instrument, secondly to devise a survey strategy for the wide field design
reference surveys that are proposed for 4MOST, and thirdly to verify that
4MOST, as designed, can indeed achieve its primary science goals. We describe
the overall structure of the 4FS, together with details of some important 4FS
subsystems. We present the initial results from the 4FS which illustrate
clearly the value of having a functioning facility simulator very early in the
conceptual design phase of this large project.
