In baseball:
There are four bases in the game: first base, second base, third base, and home plate; to score a run, an offensive player must complete, in the sequence shown, a circuit of those four bases. When a batter receives four pitches that the umpire declares to be "balls" in a single at-bat, a base on balls, informally known as a "walk", is awarded, with the batter sent to first base. For scoring, number 4 is assigned to the second baseman. Four is the most runs that can be scored on any single at bat, whereby all three baserunners and the batter score (the most common being via a grand slam). The fourth batter in the batting lineup is called the cleanup hitter. In basketball, the number four is used to designate the power forward position, often referred to as "the four spot" or "the four". In cricket, a four is a specific type of scoring event, whereby the ball crosses the boundary after touching the ground at least one time, scoring four runs. Taking four wickets in four consecutive balls is typically referred to as a double hat trick (two consecutive, overlapping hat tricks). In American Football teams get four downs to reach the line of gain. In rowing, a four refers to a boat for four rowers, with or without coxswain. In rowing nomenclature, 4− represents a coxless four and 4+ represents a coxed four. In rugby league:
A try is worth 4 points. One of the two starting centres wears the jersey number 4.
On most phones, the 4 key is associated with the letters G, H, and I, but on the BlackBerry Pearl, it is the key for D and F.
On many computer keyboards, the "4" key may also be used to type the dollar sign ($) if the shift key is held down. It is the number of bits in a nibble, equivalent to half a byte
In internet slang, "4" can replace the word "for" (as "four" and "for" are pronounced similarly). For example, typing "4u" instead of "for you". In Leetspeak, "4" may be used to replace the letter "A". The TCP/IP stack consists of four layers. In transport
Many internal combustion engines are called four-stroke engines because they complete one thermodynamic cycle in four distinct steps: Intake, compression, power, and exhaust. Most vehicles, including motor vehicles, and particularly cars/automobiles and light commercial vehicles have four road wheels. "Quattro", meaning four in the Italian language, is used by Audi as a trademark to indicate that all-wheel drive (AWD) technologies are used on Audi-branded cars. The word "Quattro" was initially used by Audi in 1980 in its original 4WD coupé, the Audi Quattro. Audi also has a privately held subsidiary company called quattro GmbH. List of highways numbered 4

In sports
In the Australian Football League, the top level of Australian rules football, each team is allowed 4 "interchanges" (substitute players), who can be freely substituted at any time, subject to a limit on the total number of substitutions.