want another baby but SO doesn't

So for my entire life I have always wanted 4 children of my own. With my first partner I had two children, a boy and then a girl(now 7&6). Now with my current partner I have my son (almost 7 months). He has two children from his past relationship as well a girl and a boy (8&6). Don't get me wrong I love his kids like they are my own but I still have this hole inside me that's really can only be filled by having another child of my own. I have my three and I thought when I got pregnant with our son that maybe that feeling would go away and I'd be ok with only having  3 of my own. But it didn't go away and I really want just one more child, but my So doesn't. I have kind of kidded around with him hinting that I'd like to have one more and he has just laughed and said yea right. I mean I can understand why he wouldn't want another child because that would be 6 kids total but I also feel like a part of me is missing. I don't necessarily want another right now but in the next couple years for sure. I know he doesn't want another because he thinks of expenses like college and cars and stuff like that as they get older. But I know him and if it just happened he wouldn't be mad or upset. He would even be excited after the initial shock lol. I just don't know what to do. Do I deny myself that fulfillment that I have dreamed of my entire life or should I beg him to give me another baby? Or I know this sounds terrible so please no bashing because I know I'm not the only one that something like this has ever crossed their minds, but should I take things into my own hands and if it happens then it happens? Any advise would be greatly appreciated. Or if there are any other moms out there in a similar situation let me know you're out there so I don't feel so bad for feeling like this.