Hi E.,
This sounds very familiar... both in terms of my son, and my husband. The truth of the matter is, your son won't go poop in the potty until he wants to. He's in control, as much as we wish we could really be. Right now, if he's like my son, he just associates going poop on the potty with stress. He wants to avoid it, because it just seems to bring out reactions he doesn't like in everyone... so he probably just sneaks off somewhere to avoid the whole thing. On the other hand, going pee is probably a no-stress deal... so he happily goes to the potty for that.
I know this hard, because I'm stuck in the very same place, but you have to gently ask your husband to back off. Explain to him that your son isn't getting positive feelings about going poop, it's all negative and stressful. Your mom too... even "helpful" involvement is just too much involvement right now.
Now try and help your son to relax about the whole thing. And just show him that going poop in the potty is something that people start to do as they get bigger. And if isn't uncomfortable for you... take him with you when you go. And give yourself a "reward" when you go poop in the potty (a pretzel stick, a froot loop... whatever). If he has an accident, just tell him... "oops, you had an accident... lets go put the poop in the potty where it belongs" and let him see you dump it there. Change him, and go on with your day. Ask you husband not to get upset.
Pick 2 or 3 times a day to see if he'll sit on the potty, just to see if any poop wants to come out. No stress... don't worry if he does anything. If nothing happens, no big deal... thank him for giving it a try. Give him a reward if you want for trying. (again, trying to get rid of the "stressful" feeling of sitting on the potty to go poop) If something does happen, then praise him big. Tell him how great it is that the poop went right where it belongs, and give him a bigger reward (3 pretzel sticks, m&ms... whatever).
And then... just be consistent. Consistent in your approach, and consistent in your emotions. I KNOW it's hard not to be frustrated, but for a little bit forget that he's four and what your expectations are of a four year old. Just focus on helping him to learn a new skill. Your husband wouldn't get angry if it took him a while to learn how to hit a baseball... right? Try and have him look at it that way.
Best of luck to you. Be patient, relax... and let your son lead the way. It will happen. :)