Four is the second square number, the second centered triangular number.
In Hinduism:
The four cardinal Virtues: 1. Non-violence, 2. Truth, 3.Purity, 4.Self-control
In Buddhism:
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