Each number is represented as a circle which contains the numbers below it.

4 contains 1 2 3 and 4 (the set contains itself)

Within the containing circle: Top left is a single circle, so it is 1. Top right is 2, so its a circle with another circle in it. Bottom left is three, so it contains both the previous symbols for 1 and a 2. And the final number is 4, so it has all the previous circle diagrams in it.