Hi & Welcome/ I appreciate your concern about your 4 year old son with chickenpox. It is not advisable for your son to do swimming during chickenpox infection. Chickenpox is highly contagious. If your child has contracted chickenpox, he/she should not go swimming or play contact sports unless all blisters have crusted over. This normally takes place a week after the first rash started. Try to avoid public places, as others may catch the virus, which may be dangerous, especially to those who are pregnant or have a weak immune system. Swimming may also soften the crusts, which may result in a more intense itch and the crusts may fall off early leaving scars. Hope to have answered your query.