When you as a Chan cultivator enter into a state where you forget yourself and forget others, you have reached the stage of the First Dhyana, which is called the Stage of the Happiness of Leaving Birth. At this stage, you have broken away from the attachments of living beings and experience the feeling of "taking Dhyana bliss as food and being filled with the joy of Dharma." In this samadhi, your breath will have stopped; you will no longer inhale or exhale. There is a unique happiness that is wonderful and inexpressible. This happiness is one that most people never experience.
The Second Dhyana is called the Stage of the Happiness of Producing Samadhi. In this samadhi, you experience the greatest joy. When you're sitting, you don't want to eat or drink. Your pulse has stopped. You're just like a dead person, but you still have thoughts and you are aware that you are sitting in meditation.
The Third Dhyana is the Stage of the Wonderful Bliss of Leaving Happiness. In this stage of samadhi, you leave the happiness of "taking Dhyana bliss as food and being filled with the joy of Dharma" and attain a kind of wonderful bliss that is impossible to describe. It's subtle and inconceivable. In this particular state, your thoughts have stopped. You have no thoughts. It's been described as: "When not even one thought arises, the entire substance appears. But when the six sense organs suddenly move, you're covered by clouds."
The Fourth Dhyana is the Stage of the Purity of Renouncing Thought. When you reach this state, not only are your thoughts stopped, they are eliminated entirely. At this time, you attain an extremely pure and wonderfully subtle joy. However, the state of the Fourth Dhyana is still at the stage of a common mortal. You have not yet realized fruition of your cultivation. Do not assume you are a special person, because you're still far from realizing the fruition. You should still work hard at your cultivation and meditate diligently, because if you can advance, you'll reach the state of the Five Heavens of No Return. That's when you enter the Dharma-nature stream of sages.
There was a person known as the Unlearned Bhikshu, who had cultivated to the stage of the Fourth Dhyana, but mistakenly assumed that he had realized the fruition of Arhatship. He went about announcing that he had realized the fruition. Because he did not have a thorough understanding of the Buddhadharma, he uttered a major lie and later fell into the uninterrupted hells.