It takes a extremely disciplined player with remarkable self control to reach stage 4, and very few players will get there. A player at stage 4 will not register any mental anguish from losing a big pot. Rather than feeling anger, frustration, or even acceptance of the hand, he will be focused entirely on how his opponents played and what can be learned from the hand. Whether he won or lost is an irrelevant detail.
(a paragraph from my most recent book. I'm pretty sure Jesus Christ himself is the only well known poker player to reach stage 4).