Why do you notice the little piece of dust in your friends eye, but you don't notice the big piece of wood in your own eye? How can you say to you friend, 'let me take that little piece of dust out your eye'? Look at yourself! You still have the big piece of wood in your eye. You hypocrite! First take the wood out of your own eye. Then you will see clearly to take the dust out of your friend's eye.
Jesus is teaching us not to look at other people and point out their faults or sins when we have faults and sins of our own. In verse 4 he is saying it is not possible for us to help others with their faults when we still have them.