When a child picks his/her nose, the lining becomes irritated and can give them an itchy sensation. This makes them pick it more. (This also happens when people use Q-tips roughly in their ears.) Then it can become a habit they do without even realizing they are doing it. Let him know you know how hard breaking this habit will be for him, but that it is important and you are there to help him. I suggest holding a mirror in front of him when he does it so he is more aware that he is doing it and sees how noticeable it is to other people. I like the idea of making him wash his hands afterwards too (for hygiene if nothing else). My sister did a reward jar for her son. She put ten pennies (it could be anything)in a jar , and each time her son did the bad habit, she took a penny out. At the end of the day, he got to keep what was left in the jar. (You may want to do a morning jar, afternoon jar, and evening jar since one day is a long time for a 3-4 year old to wait for a reward.) Also try to give him something else to do with his hands when he is tempted to pick his nose. Try reading him a book on your lap while he plays with a special toy. Distraction helps. This will probably take at the very least a month, but hopefully each day he will get better and better. Good luck.