Why does Job 25:4 say that no man born of woman can be pure if Jesus, born of woman, is pure?
Job is unique among biblical writing. It is an account of the Satanic attack on Job, on his own response, on the opinions of his friends Bildad, Eliphaz, Zophar, Job’s responses and also the advice of the younger Elihu. What makes this unique is that much of the book is the opinion of humans–humans who are wrong in their opinions. In Job, his friends tell Job that he is suffering because of his own sin. This is simply not the case. Also, Job says that he is a righteous man who does no deserve any of he suffering he is going through. This is also at least in part untrue. Although Job is inspired, it contains the opinions of men and some of those opinions are not true. Besides, Job is poetry and poetry has much imagery and symbolism which is not to be taken literally. For this reason, we need to be very careful to use the Book of Job as a source of theology or doctrine. It is a book about theodicy–about God’s sovereignty and about the cause of evil and of suffering. There is much theology here, but that theology needs to be drawn from the book carefully.
Specifically with regard to your question, in Job 25:4 Bildad is speaking his opinion here. His opinion may not reflect truth as God would give it. Besides, he does not say that one born of woman cannot be pure. In this passage he is asking a rhetorical question, not making a statement of fact. Because he is asking a rhetorical question, not making a statement, we cannot even logically infer that he is claiming that no one born of a woman can be pure. A question is not a statement.
So, can a man/human born of a woman be pure? I believe that tiny children are in fact pure. I believe that all of us are born innocent and pure. It is only later that we sin and lose our innocence. Jesus may be the only one who lived through adulthood remaining innocent, but he is not the only one who has ever been innocent.
In summary, given the genre of literature in Job, large parts of the book should not be read to determine biblical doctrine. Therefore, although your question is an interesting one, we cannot infer any particular biblical truth from this passage.