The Byron Katie "Four Questions" method, also known as The Work, is a process of self-inquiry designed to help people identify and question the thoughts that cause them stress and suffering.
The process begins by writing down a stressful thought or judgment (often about another person or a situation) and then applying the following four questions to that specific thought:

• Is it true? (The answer should be a simple yes or no.)
• Can you absolutely know that it's true? (This encourages deeper reflection beyond simple belief.)
• How do you react—what happens—when you believe that thought? (This involves noticing the emotions, sensations, and actions that arise when you hold the belief.)
• Who would you be without the thought? (This is an invitation to imagine how your life and inner state would change if the stressful thought didn't exist.)

After answering the four questions, the final step is to find a Turnaround. A turnaround is an opposite or inverse of the original thought. For example, if the thought is "He should understand me," possible turnarounds could be "He should not understand me," "I should understand him," or "I should understand myself." You are then invited to find three specific, genuine examples of how the turnaround statement is as true, or truer, than the original thought.

The entire method is intended to be a form of self-meditation and inquiry to allow the mind to spontaneously let go of its attachment to stressful beliefs.
You can learn more about this method from The four questions that can help your mind heal | Byron Katie.

This video explains Byron Katie's philosophy on suffering and introduces her four questions method.