4 is a composite number, and the first natural number with this property. 4 is an even square number. 4 is the 2nd tetrahedral number. 4 is the 2nd Busy Beaver number. Plugging 2 into the hyper operators degenerates to 4: \(2 \uparrow\uparrow ... \uparrow\uparrow 2 = 4\).
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.
In Greek-based number naming systems, 4 is associated with prefix tetra-, and with prefix quadri- in Latin systems.
Googological functions returning 4 Edit