4

4 was simplified by joining its four lines into a cross that looks like the modern plus sign.

The Shunga would add a horizontal line on top of the digit, and the Kshatrapa and Pallava evolved the digit to a point where the speed of writing was a secondary concern.

The Europeans dropped the finishing curve and gradually made the digit less cursive, ending up with a digit very close to the original Brahmin cross.

On the seven-segment displays of pocket calculators and digital watches, as well as certain optical character recognition fonts, 4 is seen with an open top: .

[2] Television stations that operate on channel 4 have occasionally made use of another variation of the "open 4", with the open portion being on the side, rather than the top.

This version resembles the Canadian Aboriginal syllabics letter ᔦ.

The magnetic ink character recognition "CMC-7" font also uses this variety of "4".

[3] There are four elementary arithmetic operations in mathematics: addition (+), subtraction (−), multiplication (×), and division (÷).

[4] Lagrange's four-square theorem states that every positive integer can be written as the sum of at most four squares.

A four-sided plane figure is a quadrilateral or quadrangle, sometimes also called a tetragon.

It can be further classified as a rectangle or oblong, kite, rhombus, and square.

Four is the highest degree general polynomial equation for which there is a solution in radicals.

[9] The largest planar complete graph has four vertices.

[11] The regular tetrahedron, also called a 3-simplex, is the simplest Platonic solid.

[12] It has four regular triangles as faces that are themselves at dual positions with the vertices of another tetrahedron.

[13] The smallest non-cyclic group has four elements; it is the Klein four-group.

They are defined as locally trivial fibrations that map

(aside from the trivial fibration mapping between two points and a circle).