System 4 may refer to:

• The fourth release of Classic Mac OS
• International Computers Limited#English Electric LEO Marconi (EELM)

• William L. Jenney
• William L. Kruer
• William L. Lane
• William L. Love
• William Lloyd (cleric)
• William Lloyd, engineer
• William Lloyd (engineer)
• William Lloyd (minister)
• William Lloyd Nelson
• William Lloyd (priest)
• William Lloyd (rugby union)
• William Lloyd Standish IV
• William Lloyd-Webber
• William L. Manger
• William L. McCaughey
• William L. McCauley
• William L. McMillan
• William L. Melvin
• William L Melvin
• William L. Merry
• William L. Moore
• William L. Moore (Virginia politician)
• William L. Mooty
• William L. Murphy
• William L. Nicholson