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