Open a checking account in YOUR name only and do NOT tell him you have it.
Pack up a few changes of clothes for you and your son and GET OUT! Do it yesterday. Do NOT leave a note telling him you are leaving, and do NOT let him know where you have gone.
Contact your son's school, and tell them that his father is NOT to be allowed to pick him up anymore. Or pull your son out of school and enroll him somewhere else, with instructions ot to releasae him to anyone but you. I assume that if he is four, it is preschool and not public school assigned by your address.
If there is a shelter in your area, go there. If not, go to a friend or family member's house, or even a cheap motel. If he finds you and shows up there, CALL THE COPS.
Loneliness is scary, but how much more lonely can you be than in the situation you're in now? And alone is better than dead, which is where you will eventually be as his abusive behavior escalates, which it will. Been there, done that, got the scars.
Your son is attached to his father because he sees that his father has all the power in the home. He is learning that the way his father treats you is the way men should treat the women in their lives, and if you keep him in that situation, it is how he will treat the women in his life when he's older, because he will think it's normal.