Analysis of information sources in references of the Wikipedia article "4" in English language version.
7 is an example of an integer that can't be written as the sum of three squares.
There is no algebraic formula for the roots of the general polynomial of degrees 5 or higher.
(i.e. That there are maps for which three colors are not sufficient)
... The complete graph on the largest number of vertices that is planar is K4 and that a(K4) equals 4.
...the smallest possible number of faces that a polyhedron may have is four
...face of the platonic solid. The simplest of these shapes is the tetrahedron...
...the tetrahedron plays an anomalous role in that it is self-dual, whereas the four remaining polyhedra are mutually dual in pairs...
The Klein four-group is the smallest noncyclic group,...
2 ↑↑ ... ↑↑ 2 is always 4
Four was a sacred number of Zia
In Chinese, Japanese, and Korean, the word for four is, unfortunately, an exact homonym for death
The Klein four-group is the smallest noncyclic group,...
