

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.



















Excellent question, the answer is you’ve now reached the end of my understanding of set theory