I think that if the following doesn't work, you may need a child therapist. His anxieties are probably not just about himself. I'm guessing he doesn't want mommy going away to the hospital anymore and that adds to his already stressful thoughts.
My advice is coming from a mom of a little boy whose dad had a brain hemorrhage, was hospitalized for 6 months and who will never be the same dad he knew. And believe me, he asked, more than once, if he is going to have one, too. Not to mention I can lean toward the anxious side, too. My son has since had 2 sports injuries to the head--golf and baseball--and it took everything I had not to let him know I was worried about him having a brain hemorrhage (while we sat in the same emergency room as we did for dad) waiting for stitches and x-ray results.
You need to backtrack a bit. You need to keep it simple and in 4 year old or younger language. He is now coming at this from the angle that "this is how some people poop and I may be one of those some people!"
If I were in this situation I would say, "Mommy was very, very sick and my pooper was broken so MY doctor made it so I could poop differently for a while. But now I don't need to poop that way anymore. YOUR doctor said your pooper is fine and will ALWAYS be fine and you can poop the normal way. Your doctor said that you do NOT need that special opening that I had and he is not going to give you that opening." Don't worry about being politically correct, don't worry about fear that you are lying to him if he gets sick one day--just get him over this hump in a simple, straightforward way. STRESS the fact that his doctor said he is fine! You do not want him to suddenly develop a fear of going to the doctor thinking he will cut an opening in him.
The best thing the ICU nurses, and later I, did for my son was to ask him if he has any questions and just spend time answering them. They spent so much time with him, I will never forget that. He knew we would keep no secrets from him. We just answered him on his level. Stop and think like an anxious 4 year old before you answer. If he says, "Will this happen to me?" you don't say, "probably not", you say "No, it will not. I was very sick and you are very healthy. I had a special kind of sickness that you will not get. And the only way to poop this way is if the doctor gives you the special opening and show him that he doesn't have that opening. Sorry, no opening so you have to poop the boring way for the rest of your life!"
The other thing that jumped out at me, from an anxious person's perspective, is the fairy garden. If it is working, fine. But my main concern here is that you are using it to help protect him and help him feel safe. A person would only need protection if there is a threat. So maybe in his little mind there is still a threat if he needs the fairies watching over him. If he is okay and normal he would not need this sort of protection. I hope I am explaining this okay. I know I am rambling.
I guess, to help your son, I wouldn't "normalize" the stoma. If he had no fears about it then I would stick to the "this is how some people poop" scenario. Your son needs to be told that the stoma is NOT the norm. He is a healthy young boy and will not need the stoma. Maybe have him name all the people he can think of like this and make it fun! He says, "Does dad have a stoma?" You say, "nope!". "Does Uncle Jimmy have a stoma?" You say, "nope, no stoma for Uncle Jimmy!" "Does Mickey Mouse have a stoma?" "No, silly, no stoma for Mickey!" And trade off. You ask, "Does our doggie have a stoma?" and let him answer and on and on.
Maybe also let him pick out a new potty or new diapers. Let him take the lead on the next step. And have dad let him watch while he goes to the bathroom, too!
I hope I was able to help! Good luck!
added:
p.s. I don't get how you can have a 4 yr old if you were pregnant for the first time in Jan of 2011?? And how would a 2 or 3 year old remember something like that from last year when they were 1 or 2??