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.
A later study of 449 U.S. Navy special warfare trainees found no significant difference in the incidence of stress fractures among sailors and Marines with different arch heights. See also
Planovalgus deformity
Marfan syndrome
Ehlers-Danlos syndrome
Comparison of orthotics
Rocker-bottom feet

References
External links
Can minimalist shoes help flat feet
Best flat feet shoes solution
Natural flat feet solution the details