Year 4 (IV) was a common year starting on Wednesday or a leap year starting on Tuesday (link will display the full calendar) of the Julian calendar (the sources differ, see leap year error for further information) and a leap year starting on Tuesday of the Proleptic Julian calendar. At the time, it was known as the Year of the Consulship of Catus and Saturninus (or, less frequently, year 757 Ab urbe condita). The denomination 4 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.

4 (four; ) is a number, numeral, and glyph. It is the natural number following and preceding . Four is the only number that has the same number of characters as its value in the English language.

In arithmetic and algebra, the fourth power of a number n is the result of multiplying four instances of n together. So:

n⁴ = n × n × n × n

Lego 4+ (also previously known as 4 Juniors in 2003) was a Lego theme for children 4 years old or more. The product range used regular Lego bricks, but came with a different kind of minifigure that were larger and couldn't be disassembled. The sets tended to have a small number of pieces, in order to be easy to put together.

Unicode has subscripted and superscripted versions of a number of characters including a full set of arabic numerals. These characters allow any polynomial,  chemical and certain other equations to be represented in plain text without using any form of markup like HTML or TeX.