Unfortunately, this is an issue with ZBrush 4. Some people have found that modifying the front and back opacity settings under Preferences>Draw lessens the
effect somewhat. It updates as you change the values, so it should be fairly quick to get a look that works if you want to go that route.
I too have this problem.
Neither of these solutions work for me.
Did an internet search and found that a lot of people have problems retopology problems in ZBrush.
Has anyone found a definitive solution for this?