4

The number four (4 in decimal form) is the fifth non negative integer/natural number, the fourth positive integer, and the integer following 3 and preceding 5 on the number line. It is the second square number, the second power of two, and the second tetrahedral number.

It is simple to picture four objects in one's head, although it is not as easy as picturing 3, 1, 2, or 0 objects.