Four-colour problem a mathematical problem to prove that any plane map can be coloured with only four colours so that no two same-coloured regions have a common boundary.
four eyes see more than two two people are more observant than one alone (compare two heads are better than one). The saying is recorded from the late 16th century.
four freedoms the four essential human freedoms as proclaimed in a speech to Congress by Franklin D. Roosevelt in 1941: freedom of speech and expression, freedom of worship, freedom from want, and freedom from fear.
Four Horsemen of the Apocalypse War, Famine, Death, and Pestilence; they are traditionally identified with the riders of the white, red, black, and pale horses seen in Revelation.
four last things in Christian belief, death, judgement, heaven, and hell, as studied in eschatology.
four-letter word any of several short words referring to sexual or excretory functions, regarded as coarse or offensive; the term is recorded from the 1920s.
four noble truths the four central beliefs containing the essence of Buddhist teaching; they are that human life is characterized by frustration and suffering, that the cause of this is desire and greed, that desire must therefore be got rid of, and that following the eightfold path is the way to achieve this.
see also four-leaved clover, First Four Ships at first, there is more to marriage than four bare legs in a bed, the four winds at wind.