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.
(An exception to this rule is the Super League, which uses static squad numbering.) In rugby union:
One of the two starting locks wears the jersey number 4. In the standard bonus points system, a point is awarded in the league standings to a team that scores at least 4 tries in a match, regardless of the match result. In other fields
The phrase "four-letter word" is used to describe many swear words in the English language. Four is the only number whose name in English has the same number of letters as its value. Four (四, formal writing: 肆, pinyin sì) is considered an unlucky number in Chinese, Korean, Vietnamese and Japanese cultures mostly in Eastern Asia because it sounds like the word "death" (死, pinyin sǐ). To avoid complaints from people with tetraphobia, many numbered product lines skip the "four": e.g. Nokia cell phones (there was no series beginning with a 4 until the Nokia 4.2), Palm PDAs, etc. Some buildings skip floor 4 or replace the number with the letter "F", particularly in heavily Asian areas. See tetraphobia and Numbers in Chinese culture. In Pythagorean numerology (a pseudocience) the number 4 represents security and stability. The number of characters in a canonical four-character idiom. In the NATO phonetic alphabet, the digit 4 is called "fower". In astrology, Cancer is the 4th astrological sign of the Zodiac. In Tarot, The Emperor is the fourth trump or Major Arcana card.