Four is the second square number, the second centered triangular number.
(Thank you to Seth for send me the numbers)
The four cardinal Virtues: 1. Non-violence, 2. Truth, 3.Purity, 4.Self-control
The Group of Fours 1. The Four Noble Truths 1. Suffering 2. The Cause of Suffering 3. The Cessation of Suffering 4. The Way Leading to the Cessation of Suffering