Four is a good age to learn to swim, as this is when he is mature enough to be with a swimming instructor in the water without you. He should also be able to be in the water without swallowing a lot of water, should be able to follow instructions, and should also be able to stop doing something when told to.
Find a certified swimming instructor in your area who gives lessons to small children. As it is important that the lessons be fun for your child, ask if the lessons include music or games. It is also important that your child be allowed to learn at his own pace. Do note that it is quite normal for a child to stay in the beginner’s class for more than one session, because the first couple of times might be to simply become used to the water.
Make sure that there is enough supervision - usually one instructor for a maximum of eight children. Also, as your child is a beginner, these classes should include water safety, basic floating, breathing, as well as leg and arm movements.