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

PropertiesEdit

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.

4 is associated with bad luck in Eastern numerology because the Chinese word for "four" sounds similar to the word for "death."

It is possible to test for divisibility by 4 in base 10 by looking at last 2 digits of the number. If the 2-digit number is divisible by 4, then the whole number is.

Ignoring weekdays, the Julian calendar repeats every 4 years.

Its Roman numeral is most commonly IV, but on clock faces, IIII is also used.

In googology Edit

Some googologisms based on 4 are supertet, quadroogol and googolquadriplex. 4 can be thought as zeroth term of sequence defining Graham's number: \(g_0\) = 4. 

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

4 can be named gartwo, fztwo, fugatwo, megafugatwo, boogatwo, or twoplaton with the gar-, fz-, fuga-, megafuga-, booga-, and -platon prefixes respectively.

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.