In religion
Buddhism
Four Noble Truths – Dukkha, Samudaya, Nirodha, Magga
Four sights – observations which affected Prince Siddhartha deeply and made him realize the sufferings of all beings, and compelled him to begin his spiritual journey—an old man, a sick man, a dead man, and an ascetic
Four Great Elements – earth, water, fire, and wind
Four Heavenly Kings
Four Foundations of Mindfulness – contemplation of the body, contemplation of feelings, contemplation of mind, contemplation of mental objects
Four Right Exertions
Four Bases of Power
Four jhānas
Four arūpajhānas
Four Divine Abidings – loving-kindness, compassion, sympathetic joy, and equanimity
Four stages of enlightenment – stream-enterer, once-returner, non-returner, and arahant
Four main pilgrimage sites – Lumbini, Bodh Gaya, Sarnath, and Kusinara

Judeo-Christian symbolism
The Tetragrammaton is the four-letter name of God. Ezekiel has a vision of four living creatures: a man, a lion, an ox, and an eagle. The four Matriarchs (foremothers) of Judaism are Sarah, Rebekah, Leah, and Rachel. The Four Species (lulav, hadass, aravah and etrog) are taken as one of the mitzvot on the Jewish holiday of Sukkot. (Judaism)
The Four Cups of Wine to drink on the Jewish holiday of Passover. (Judaism)
The Four Questions to be asked on the Jewish holiday of Passover. (Judaism)
The Four Sons to be dealt with on the Jewish holiday of Passover. (Judaism)
The Four Expressions of Redemption to be said on the Jewish holiday of Passover. (Judaism)
The four Gospels: Matthew, Mark, Luke, and John. (Christianity)
The Four Horsemen of the Apocalypse ride in the Book of Revelation.
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.