S4

S4, S 4, Š-4, S.4 or S-4 may refer to:

; beginning with S4 :
* S4, a highly accurate 4mm/1 ft railways modelling format
* S4, a sustainability trends in fashion newsletter created by model Summer Rayne Oakes and found at [http://www.S4trends.com/ S4trends.com]
* S4, the normal modal logic
* S4 algebra, a variety of modal algebras, also called Interior algebra
* S4 (Berlin), a S-Bahn line
* S4 inc., Burlington, Massachusetts, top secret facility clearance mission-critical support services to customers like the Department of Homeland Security and U.S. Air Force [http://www.s4inc.com/index.html]
* , a safety phrase in chemistry
* S4 League, a Korean MMORPG Shooter made by Pentavision, the makers of the music video game, DJMAX
* S4 (military), a logistics officer within military units
* S4 (Munich), a S-Bahn line in Munich
* S4 postcode, covering areas of northern Sheffield

; containing S4 :
* Alco S-4, a locomotive
* Audi S4, a car
* British NVC community S4, a swamps and tall-herb fens community in the British National Vegetation Classification system
* County Route S4 (California)
* Form S-4, a [http://www.sec.gov/about/forms/forms-4.pdf filing] required by the Securities and Exchange Commission to declare Mergers and Acquisitions. [http://www.investopedia.com/terms/s/s4.asp]
* Lancia Delta S4, a 1984 Group B rally car
* Letov Š-4, a 1923 Czechoslovak single-bay unstaggared biplane fighter
* Nikon Coolpix S4, a 6.0 million pixels digital camera
* Prussian S 4, a 1894 steam locomotives class
* Seibel S-4, a 1951 helicopter
* Siemens S4, a Siemens mobile phone
* Supermarine S.4, a 1925 British single-engined single-seat monoplane racing seaplane
* USS S-4 (SS-109), a 1919 US Navy submarine

S₄ may refer to :
* Fourth heart sound, an abnormal heart sound often indicative of congestive heart failure or cor pulmonale
* tetrasulfur, an allotrope of sulfurS04 may refer to :
* S04, a code name for FC Schalke 04

S4 is a designation or shortening for :
* Advanced Configuration and Power Interface (ACPI), S4 refers to hibernation, a sleeping state in which the contents of RAM are saved to a hard disk
* SATA International IATA airline designator
* Students for saving social security
* Version 4 of S (programming language)
* sacral spinal nerve 4, a spinal nerve of the sacral segment
* Fourth sacral vertebra of the vertebral column, in human anatomy
* Sector Four, an alleged military base next to Area 51, said to contain nine UFOs
* a Yamaha grand piano model
* a planned Bremen S-Bahn line
* a Hanover S-Bahn line
* a RheinNeckar S-Bahn line
* a Rhein-Ruhr S-Bahn line
* a Rhine-Main S-Bahn line
* a Rostock S-Bahn 2010-planned future line
* a Stuttgart S-Bahn line
* a Zürich S-Bahn line
* a Stadtbahn Karlsruhe line