I was once told this "you'll never regret the child you have, but you'll always regret the child you didn't." Your situation sounds eerily similar to mine, I guess I would have to agree that I have this strange "obsession" as you call it, with having a 4th, but my husband isn't on board. And I am having a hard time not resenting him for it. Which was the catalyst for the rockiness in our relationship.
Yes, I think it is totally normal to want more kids, just as I think it is totally normal to not want anymore or know when you are done, or want none. Why do some of us want more than others? Different strokes for different folks, i guess! My aunt has 4, another aunt has chosen to have no children, a friend has 8 and most of the rest of my family and friends have 2. I have 3 but want 4. I have some ideas of why I want more, but really that is irrelevant. And in my opinion you don't have to justify why to anyone. My reasons mean nothing to my husband, we will never see eye to eye on it.
It sounds to me like you are perfectly aware of your financial and marital situation. You have also made it abundantly clear you are very grateful for the 2 healthy children you do have. And I'm pretty sure you are wise enough to know a baby is not going to perfect your marriage. I don't post on here much so I am not aware of the back story with you and your husband. And maybe that is where many of these mamas are coming from. Although, I don't think being made to feel guilty has any place in this discussion. Yes, there are plenty of people out there who would do anything to have just one baby, but that is not your issue, nor should that factor into your decision or stop you from living YOUR dream. It think that assumption/suggestion is simply asinine. I have my teaching credential, and I am very proud of myself for that but I would like to get my masters. So if I used the same reasoning suggested on here... because it would be tight financially, and because there are people out there who have not even a high school diploma, I shouldn't go after my dream? That's just crazy talk, as are some of these responses!
As for a rocky marriage, what marriage isn't people? I don't know one person who has a marriage any better than mine. None. Not my grandparents married 65 years, my parents married 43, my aunt and uncle married 35 with 4 kids, not my aunt and uncle who have no kids, not my best friend, not my sister, not my neighbor, not my kids teacher (who is also my coworker and friend). Seriously, marriages are tough and if we didn't have kids because marriages got rocky, then there would be far less people in this world! Which might not be a bad thing! ;) I'm actually surprised people still get married!
People talk like divorce is just the worst thing you can do to kids. Sure it's not ideal. Of course you don't go into a relationship/marriage and create a family thinking this is an option. But if a 3rd child sends your marriage over the edge, so be it. Kids have been surviving divorces for quite some time now. It's better than death. My friend just lost her husband at the age of 36. She has 2 kids under age 6. Their dad will never be in their life. Divorce isn't death. Of course you don't want that, but come on people, kids are resilient. We are resilient. We are all screwed up in some way, divorce or not. Bottom line, this is you and your husbands decision, you are both on board and you're not getting any younger, I would go for it! Good luck!