Little Boys pulling their penis

Hi, My son is 4 in February and for the first time today I walked in to the front room and he had his pants and trousers down pulling his penis as soon as he saw me jumped up. When I asked what he was doing he said nothing and laughed. I was cooking dinner this evening so put the tv on I went in to front room again and he done it again. When I asked again he had fluff in the end and laughed. I really don’t know how to deal with the situation he is my first child, if anyone has any advise I would be really grateful. Thank you