System 4
System 4 or System IV may refer to:

Acorn System 4, the early minicomputer
The fourth release of Classic Mac OS
International Computers Limited#English Electric LEO Marconi (EELM), also known as ICL System 4
Not-released Unix system between UNIX System III and UNIX System V
STS-4 (Space Transportation System-4), the Space Shuttle mission

See also
IBM System/4 Pi