It may be hormones or more likely that you have a need to be "needed". Your additional response suggests that you only want to hear from people who will confirm your feelings on the topic, but you really need to think about this. I can't help but think like a "pshrink", so just bear with me on this one.
You are a SAHM who has a son who will be a full-time student in September, assuming he is in 1/2 K somewhere. Your children don't need you in the same way that they once did, so you may be looking for something or someone to fill that void. Because you are a career mom, that feels like the natural solution.
Just a thought... maybe a part-time job or a volunteer position somewhere? My mother went through this when my youngest sister started school and found a volunteer position at a hospital working in the neonatal unit and in family relations. She loves it and 15 years later still works there two days a week! She had to feel needed and did not have to work, so spending time with people who needed someone to talk with, cry with and just laugh with really helped her. It also gave her something to talk about with my dad and her friends aside from us!
Just another thought... are you sure that this is the original reason for the vasectomy? This procedure is not meant to be temporary and I would be surprised that a doctor would perform the procedure under that reasoning. How does your husband feel about this idea? You didn't say-
If you do elect to reverse the vasectomy, please don't go with a "cheap" doctor. I don't think that this is something to scrimp on!
Keep in mind that you are talking about starting over... diapers, late nights, etc with three older and likely very active children who will continue to put demands on your time and energy.
I know that there are people who will jump all over this, but there is a serious financial impact here. Aside from the $$$ for the procedure, you are also talking about buying all new "gear". I doubt you saved everything from your previous children at this point. You are also now talking about supporting FOUR children- braces, sneakers, food, clothes, COLLEGE TUITION. If you are worried about the cost of the procedure, I worry that having another child will add a significant financial burden to your family. It sounds like you don't want to hear that part, but it's true.
It could take you a while to get pregnant and there is no guarantee that the reversal will work. Take some time to think this through b/c the source of your feelings may be different than wanting another child.