One thing that Job wants to remind his friends of in Job 21:4 is that his complaint is not against them. It’s against God.
4 As for me, is my complaint [to/again a] man?
[and if it were so/and why/if so], why should [not my spirit/I not] be [troubled/impatient]?
And I don’t know about you, but when someone under my authority complains about something – really, anything – a strange thing can happen. I can assume that that person is complaining about me.
She could be complaining about anything – even something that has nothing to do with me. And yet, when there’s a complaint, I can easily think that the complaint is directed against me.
And I wonder if that’s what Job is getting at here. He’s wanting to assure his friends that his complaint is not at all against them – or, at least it wasn’t originally. The friends aren’t Job’s real problem. Job’s problem is with God himself.
So, Job is trying to assure the friends that this is nothing personal against them. He’s having problems figuring out why God is seeming to punish him even though he’s righteous.
So, in other words, he’s telling the friends to back off. Job’s complaint and impatience is directed against the Lord.