Actually, 4 sides is 5 points.

`1--------2`

5 |

| |

| |

4--------3

Each point is defined by a coordinate. Each coordinate is two ordinates (numbers), an X ordinate and a Y ordinate, so 0,1 is the first point, 2,3 the second, 4,5, the third, 6,7 the fourth, and 8,9 is the fifth which duplicates the first so that the boundary is closed. Does that make sense to you?