Eliphaz views Job as nothing more than a wind bag (cf. 15:2). In other words, he thinks Job's arguments lack substance. He views Job's every word as useless or "unprofitable talk" (15:3). Certainly Job is disappointed once again. His arguments have not been seriously considered by his friends.
Eliphaz's point in 15:4 might be this: "How can we pray or think about spiritual things when we are sitting in the presence of someone who is so irreverent to God?" Job had accused his friends of an improper attitude back in 13:7-12, and now Eliphaz charges him with such. Let it be remembered that in the end, the friends of Job were the ones who were guilty of speaking incorrectly (cf. Job 42:8).
Job had asked for proof of his sin. Eliphaz responds - "Your own mouth condemns you, and not I; yes, your own lips testify against you" (Job 15:6). Eliphaz claims that Job's very words are enough proof that he has a serious sin problem. No other evidence is needed in their estimation! Sadly, Job's friends had already made up their mind about him, and there was nothing he could say to sway them (even though he tried). They saw and concluded what they wanted to about his dreadful suffering. Eliphaz's statement here (and the attitude that prompted it) reminds me of Matthew 26:64,65 where Jesus affirms His deity and is immediately charged with blasphemy. His accusers didn't want it to be true, and would not believe no matter what evidence was presented. Their ears were closed (as were the ears of Job's friends). Thankfully, unlike the religious leaders who condemned Jesus, the friends of Job do repent later and are forgiven (cf. Job 42:8-10).
Eliphaz then appeals to the voice of experience in Job 15:7-10:
"Are you the first man who was born? Or were you made before the hills? Have you heard the counsel of God? Do you limit wisdom to yourself? What do you know that we do not know? What do you understand that is not in us? Both the gray-haired and the aged are among us, much older than your father."
These ad hominem attacks against Job's age prove nothing, and they don't deal with the substance of Job's argument. Eliphaz assails Job with a barrage of questions, intended, no doubt, to humiliate him. Interestingly, this is the same basic approach God will take in chapters 38-41, but with a much different reaction from Job. God's questions humble Job, but Eliphaz's questions anger him.
Eliphaz continued in Job 15:14-16 - "What is man, that he could be pure? And he who is born of a woman, that he could be righteous? If God puts no trust in His saints, and the heavens are not pure in His sight, how much less man, who is abominable and filthy, who drinks iniquity like water!" It is true that all humans are sinners, and that many participate in sin as freely as they breathe oxygen. Nevertheless, this doesn't answer Job's argument. Job has not claimed to be utterly sinless, but he denies that he has committed anything that should result in the suffering he has endured.
The second half of the chapter (verses 17-35) records Eliphaz's long, detailed description of how God portrays Himself to the wicked man. Some believe this was a quotation from the fathers which had been handed down through the generations. Of particular interest is Job 15:20,26,27 - "The wicked man writhes with pain all his days...For he stretches out his hand against God, and acts defiantly against the Almighty, running stubbornly against Him." Eliphaz seems to be implying that Job is a wicked man who is suffering for rebellion against God.
Although Eliphaz speaks much truth, his application to Job is in error. Job's main question for his friends still remains unanswered: "Why do the wicked sometimes prosper and the righteous sometimes suffer?"