As you forgot to state which play you are studying the fourth scene of, we can only take a look at Shakespeare's Sonnet 129. Sonnet 129 very vividly describes desire through lust as a form of emotional agony and torture. Shakespeare makes it very clear he is speaking of the human emotion lust, which is a form of desire, in the very first couple of lines: "The expense of spirit in a waste of shame / Is lust in action." The phrase "expense of spirit" refers to sexual release, which he sees as a release of lustful desires and as shameful. He continues the poem to liken lust to a form of addiction: The more you want the sexual desire you crave, the more you are driven mad by wanting it, and once you get it, the more you hate yourself for having wanted it in the first place and for now wanting it even more, which drives you even more mad. Hence, in the third-to-last line, Shakespeare is calling lust both a "bliss" and a "woe," as we see in the line, "... A bliss in proof, and proved, a very woe." Therefore, we can see that Shakespeare's controlling idea about desire is that desire is both a blissful, meaning happy, feeling and a torturous emotion that brings emotional pain and shame.
One literary device, or element, Shakespeare uses to clearly portray desire in the form of lust as both a feeling of bliss and a torturous feeling is metaphor. In his final line, Shakespeare uses two metaphors to liken desire to both heaven and hell, as we see in the lines, "... yet none knows well / To shun the heaven that leads men to this hell." Here, he is using both the words "heaven" and "hell" to refer to desire. Since heaven is known to be the happiest place possible, he is using the word "heaven" to liken desire to heaven, thereby calling desire a happy state of mind. Similarly, since hell is known to be the most torturous and miserable place possible, he is clearly using the word "hell" to liken desire to hell, thereby calling desire a torturous emotion. Furthermore, since Shakespeare doesn't use the word like but rather calls desire "heaven" and "hell," we see that his final line is a perfect example of a metaphor.