Job 11:4 (ESV)

4 For you say, ‘My doctrine is pure, and I am clean in God’s eyes.’

The content of Job’s mockery, as Zophar understands and summarizes it, is that he keeps saying to God that he is pure and clean.

Eliphaz had argued, based on a nightly vision, that no one is pure in God’s sight (Job 4:17). Then Bildad had defended God’s righteousness throughout human history (Job 8:8–22). Still Job continued to assert his innocence (Job 9:21; Job 10:7; Job 10:15). To Zophar this is proud mockery.