But, you "lived through" the night. Remember that for "next" time; nothing bad actually happened.

Sorry you didn't have any support last night. I tend to read good "escape" novels for me (science fiction/fantasy, young adult, some romance, etc.) when I'm feeling too anxious.

Criticize/poke holes in your fantasies of what could go wrong; actually picture your 4 year old trying to climb the fence in the middle of the night (after getting the heavy, locked door open without waking anyone) and think about yourself at 4 and your son's actions at other times and seriously wonder, "why would he do that?" Children have reasons too, they don't just arbitrarily do things because of "shiny objects," etc. They know when things are wrong and get frustrated easily with multiple difficulties (like locked doors and high fences).

Think of something you can do between now and the next time to help you feel better next time. Enroll your son in a swim class? My grandmother was in charge of watching me when we went to the pool when I was that age and taught me to "doggy paddle" from her chair at the side of the pool (she never put on a bathing suit, was always in a dress :-) Get in the water with your son and see how he "feels" about water; it's often a bit scary to kids, the depth and all. No one likes to get water in their mouth/not be able to breathe, etc. Get some "experience" with your son in the situations you are going to leave him in so you won't worry so much/will be better able to laugh at yourself/your fears.