A: Age 4 is a rather phobic period, and children are often fearful of one thing or another for a period of time. There is probably a hereditary component to anxiety, so your son's fears are not too surprising.
Your goal is to reduce the emotional intensity that his fears stir up in him and in you. The more you put your foot down and make a big issue of his fears, the more you may add to his insecurity. Instead, strike a balance between an intense refusal to "feed into it" and an intense eagerness to protect him from experiencing any worries at all. A calm middle ground is best. A kind, casual, friendly approach can help you adjust to each situation wisely--if it is no big deal to go with him to the bathroom, nothing is gained by refusing to do so. As you walk to the bathroom, provide some realistic support by saying, "There's nothing to worry about in here, so I'll just stay right by the door and keep you company while you use the bathroom." Try to find a compromise so that he stretches his comfort zone, but only for a little bit.
Many children become anxious at bedtime. If you don't want your son to sleep in your bedroom, try staying in his room until he falls asleep. Tuck him into bed, turn out the light, and tell him that you will sit quietly in a nearby chair while he falls asleep. You can remind him that this is not a time for games, books, conversations, or drinks of water. Once your son is asleep, quietly leave the room. This approach often settles down a worried child because the parent is still close to provide a reassuring presence.
While your child's fears are real to him, he can gradually outgrow them and mature through the extra (but not smothering) support you provide.