Hello!
I think you are obessing over something you can't have. Just like a high school crush - your parents don't like the boy - so you like him even more. How do you get over it? You find a counselor who deals with grief therapy and move on.
You focus on the fact that YOU and YOUR children along with your marriage are happy, healthy and in tact. You didn't suffer a loss, not like the death of an infant - you have suffered the loss of a dream. Now, you need to thank God you have what you have and press on.
If you can afford more kids - great! Go adopt one. I am NOT trying to be mean or harsh - but this is the reality - you had a high risk pregnancy. You want another child? SERIOUSLY?! risk your life for another baby?! ADOPT!!!! Get over the vasectomy and move on with your life. If you want another child - adopt!! But STOP going over something from the past.
Yes, you can reverse a vasectomy - but the chances of it working are low. Even with today's technology. Why on earth you would jeopardize your life and that of an innocent child just to "have one more"? Seriously.
You need to seek counseling for your grief. Four years is long enough. You need to come to the realization that your life and that of an innocent child are at risk. You need to find someone who will help you deal with your grief and find a good outlet for it.
You have THREE children and a husband - more than many have. We wanted 4 - God only blessed us with two. I lost 3 - one at 20 weeks. It took a while - but the pain in less every day. I think of her every day but take solace knowing that God is taking care of her.
Live in the here and now. Live and love your children that are HERE. Thank God that you have them and a husband. Put your grief into something else but 4 years is WAY TOO LONG!!!!
talk to your pastor or priest. Best to you!!