Four is the smallest composite number, the second smallest square number, the first non-Fibonacci number, the smallest Smith number, and the smallest number that can be written as the sum of two prime numbers. Four is the number of dimensions that make up spacetime (three of space and one of time). It is the most number of colors needed to color any map so that no two neighboring areas are the same color (see four-color problem). There are four cardinal points on the compass, four Riders of the Apocalypse, and four Gospels.