4

There are 4 main blood groups in the ABO system (worldwide; frequencies differ sharply by population). A, B, AB and O, each either rhesus positive or negative. O negative is the universal donor for red cells because it carries none of the antigens the others would react to.

The temperature a domestic fridge should hold is below 4 degrees Celsius (United Kingdom guidance). The danger zone runs from about 8 to 63 degrees, so the target is set well under it rather than at its edge.

Greek natural philosophy organised matter into four elements — earth, water, air and fire — a scheme set out by Empedocles and developed by Aristotle. It passed through Islamic scholarship into medieval European medicine and alchemy, where it was paired with the four humours, and it remained the dominant account of matter until early modern chemistry displaced it.

There are 4 bases in DNA (all known cellular life). Adenine, cytosine, guanine and thymine. Three bases specify one amino acid, which is why the genetic code has 64 codons for 20 amino acids and several spare.

There are 4 chambers in the human heart (humans and other mammals and birds).

There are 4 items in working memory under stricter modern estimates (later cognitive research). Nelson Cowan's review argued the limit is closer to four when rehearsal and chunking are controlled for.

Typically, there are 4 items people can count at a glance without counting them (subitizing, across studies). Beyond about four, response time rises with each additional item, which is the signature of counting rather than perceiving.

There are 4 limbs on a tetrapod (tetrapods, including amphibians, reptiles, birds and mammals). Wings, flippers and hands are all modifications of the same four-limbed body plan.

There are 4 fundamental interactions known in physics (modern physics). Gravity, electromagnetism, and the strong and weak nuclear forces. Electromagnetism and the weak force are already understood as two aspects of one electroweak interaction, so the count is a description of current theory rather than of nature.

There are 4 Galilean moons of Jupiter (the Solar System). Io, Europa, Ganymede and Callisto, seen by Galileo in 1610. Jupiter's full moon count has been revised upward many times and will be again; these four are the durable answer.

There are 4 compartments in a cow's stomach (cattle and other ruminants). Usually described as four stomachs, and it is one stomach with four chambers — rumen, reticulum, omasum and abomasum. Only the last digests in the way a human stomach does; the others ferment.

The distance to Proxima Centauri is about 4 light-years (the Solar neighbourhood). Light leaving it today arrives here in the mid-2020s plus four years, which is as close as another star gets.

The concrete produced per person each year is about 4 tonnes. Figure as of 2023. Concrete is the most used manufactured material on Earth by mass, and cement production accounts for roughly 8 per cent of global carbon dioxide emissions.

There are about 4 billion tonnes of cement made worldwide annually. Figure as of 2023. Making it accounts for roughly seven per cent of global carbon dioxide emissions, most of that from the chemistry of the process rather than the fuel.

The average distance from the Sun to Neptune is about 4 billion kilometres (the Solar System). 4.5 billion kilometres, thirty astronomical units. Neptune takes about 165 Earth years to go round once, so it has completed one orbit since its discovery in 1846.

The altitude of the International Space Station is about 4 hundred kilometres (low Earth orbit). Low enough that the remaining trace of atmosphere slows it down, so the station has to be boosted back up several times a year.

The power output of the Sun is about 4 × 10²⁶ watts (the Solar System). 3.8 × 10²⁶ watts, of which the Earth intercepts about half a billionth.

4 is the atomic number of beryllium, a light, stiff metal that is transparent to X-rays — which is why it is used for the windows of X-ray tubes. Its dust is seriously toxic, so it is handled under tight controls.

There are about 4 billion addresses available in IPv4. Two to the power of 32, allocated out completely in 2011, which is why IPv6 exists.

There are 4 inks in the CMYK process used for most colour printing.

There are 4 letters in an ICAO airport code (worldwide, ICAO). The codes used by air traffic control, and regionally structured where the IATA codes are not: the first letter is a world region, so every airport in the United Kingdom begins EG.

There are about 4 billion airline passenger journeys flown each year. Figure as of 2023. Journeys rather than people: a return trip with a connection counts as four. Somewhere between five and fifteen per cent of the world's population has ever flown.

In Mandarin, Cantonese and other Chinese varieties, 4 (四, sì) is a near-homophone of the word for 'death' (死), leading to widespread avoidance of the number in building floor numbering, phone numbers and gifts across Chinese-speaking, and to a lesser extent Japanese and Korean, contexts.

Four beats to the bar is so dominant in Western popular music that 4/4 is called common time and can be written simply as C. Rock, pop and most dance music assume it, which is why a listener can usually clap along to an unfamiliar song without being told.

4 July is Independence Day, observed in the United States. Commemorates the adoption of the Declaration of Independence by the Second Continental Congress on 4 July 1776. The vote for independence itself was taken on 2 July; the 4th is the date on the adopted document.

A superstition in Chinese-, Japanese- and Korean-speaking cultures holds that four is avoided because its spoken form is close to the word for death, and buildings often omit a fourth floor (East Asia and its diasporas). Tetraphobia is documented in building numbering, phone numbers, hospital rooms and product model numbers. The homophony is real in Mandarin, Cantonese, Japanese and Korean, which is why the avoidance crosses all four languages rather than following one. This is a scholarly account rather than the tradition's own framing.

Presented as belief or tradition, not as an established fact.

Folklore in Irish and British folk belief holds that a four-leaf clover is said to bring luck, being a rare variant of a three-leaf plant. The rarity is real and roughly one in five thousand, which is unusual for a superstition: the thing said to be lucky is genuinely uncommon rather than merely designated. This is a scholarly account rather than the tradition's own framing.

Presented as belief or tradition, not as an established fact.

In Hinduism, the Vedas are counted as four — Rigveda, Yajurveda, Samaveda and Atharvaveda. They were transmitted orally with extraordinary precision for centuries before being written down, using recitation schemes designed to make errors detectable.

In Buddhism, the teaching opens with four noble truths. Suffering, its origin, its cessation, and the path to its cessation. The fourth truth is itself the eightfold path, so the two counts are nested rather than alternatives.

Presented as belief or tradition, not as an established fact.

In Hinduism, life is divided into four stages and society described in four varnas. The ashramas are student, householder, forest-dweller and renunciant. The varna scheme is a textual category and maps only loosely onto jati, the thousands of actual endogamous groups — conflating the two is the commonest error made about it.

Presented as belief or tradition, not as an established fact.

The age of the Earth is roughly 4 billion years. The accepted value is 4.54 billion, known to about one per cent. Dated from meteorites formed at the same time as the planet rather than from any Earth rock, because plate tectonics has recycled the oldest crust.

The highest wind speed recorded at the surface is about 4 hundred kilometres per hour (worldwide, excluding tornadoes). 408 km/h in a tropical cyclone at Barrow Island, Australia, in 1996. Tornado winds are estimated higher but are measured indirectly, by radar rather than by an instrument that survives.

There are 4 movements in a standard classical symphony (the Classical period; earlier and later symphonies vary). Fast, slow, dance, fast — a shape settled in Haydn's hands and broken freely afterwards. Several of the most performed symphonies have five movements or run their last two together.

4:3 — four units wide to three tall — was the standard screen shape for most of the twentieth century, inherited from 35mm silent film and carried into broadcast television and early computer displays. Its replacement by 16:9 widescreen is why archive television shows black bars at the sides on a modern set.

Four colours suffice to colour any map drawn on a plane so that no two regions sharing a border take the same colour. Stated in 1852 and unproved for over a century, the four colour theorem was settled by Appel and Haken in 1976 — the first major theorem whose proof depended on computer checking of cases, which made it contested on methodological grounds for years afterwards.