In 1 Peter 4:18, what does Peter mean by the righteous having difficulty to be saved? And if it is with difficulty that the righteous is saved, what will
  become of the godless man and the sinner? (1 Peter 4:18, NASB).


It seems that Peter wasn't talking about salvation from hell here because the subject is already righteous, but he's clearly talking about salvation. So, what sort of salvation is he talking about?
I would not separate this 'salvation' from salvation from hell, for I do not think there is any other kind of salvation. Sin, hell, the world and the devil are all arrayed against the believer making our death to the world and our life in Christ a target of conflict; an inwardly felt weakness and struggle. This target (the faith of believers) is the end direction of all the energy, malice and aggression that the Devil and all the whole world enslaved to sin aims at, making our salvation very difficult. Not that we can ever loose it but that it is a road of conflict by nature of the evil in this world and the remans of sin within our own selves.

On the other hand, sinners often have an easy life for their lusts are harmonious with the desires of the whole world.  If a sinner has wealth, or very good looks or a very reputable name in business or religion, he may live an easy life without intense inner conflicts against sin, for he freely follows his lusts without conflict in the world and is praised for it by the world.

The statement is saying that if a real believer who 'feels so weak' and is often actually 'externally weak' in terms of worldly power, and who 'struggles daily' like a 'bruised reed and a smoldering wick', if they are saved through this kind of 'difficulty', how will the wicked find the final judgment to be after their life of ease and lack of struggle?

The answer is clear, sinners who have no struggle with a world of sin will find hell swallowing up their souls in sudden terror, whereas the struggling weak and tortured believer, whose faith seems always tested from without and within due to our backsliding hearts, will enter at last into glory forevermore. What a contrast! This is the point of it, a contrast of life in this world and one's corresponding destiny hereafter.