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Four (4) is a positive integer one more than 3 and one less than 5.

## Properties

Four is the first even composite number. The next composite number is 6.

4 is the second square number (between 1 and 9). It is 22, and is therefore also a power of two (between 2 and 8).

4 is the second tetrahedral number (between 1 and 10).

4 is the first number not to be a Fibonacci number. It is, however, a Lucas number (between 3 and 7).

4 is the third highly composite number, being the first with 3 or more factors. The next one is 6.

4 is the first number to be a semiprime number]]. The next is 6.

## In googology

In Greek-based number-naming systems, 4 is associated with prefix "tetra-", and with prefix "quad-" in Latin systems.

4 can be named "fztwo", "fugatwo", "megafugatwo", "boogatwo", "twoplaton", "gartwo", "bartwo", "trartwo", "quadrartwo", "quintartwo", "sextartwo", "septartwo", "octartwo", "nonartwo".

Testing if a number is a multiple of 4 can be done by seeing if the last two digits make a multiple of 4. So 57368 is a multiple of 4 since 68 = 4*17.

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