What is the fourth pillar of Islamic faith?

The Fifth Pillar of Islam (Pillar of Faith) is the Hajj. Every able bodied Muslim should go through the Hajj at least once in his/her life if he/she can afford it physically, safely, and financially. The Hajj is a pilgrimage to Mecca. When in Mecca, the pilgrim must take part in various rituals, such as walking around the Kaba seven times and other rituals that can be referred to in the related question listed below.