Your question was directed toward moms of 4 or more and since I actually do fit that description I wanted to reply.
I've re-read your OP and I didn't see anywhere your asking for anyone's "personal feelings" on whether or not you should have another child so I will not give mine. I would, however, like to tell you about my own experience as a mom of 4. We are, like the previous posters, very very happy to have our 4 children. We would not have it any other way. We all have so much fun together and our kids are never bored ( they have live-in best friends!). I didn't have the chance to think it over (DD was a surprise) like you are doing, but while pregnant w/ her my "selfish side" (read: hormones) also thought about what we wouldn't be able to do as a large family. (My other worry was that people would frown upon me and see me as some kind of irresponsible, out of control woman who procreates like a rabbit. I can see that this was not a totally irrational worry.) But it's funny, DD is 2.5 now and I can't think of anything that we are missing out on because of her arrival. Rather, she has brought us immeasuralble joy that we could never place a dollar value on. Anyway, I too really thought seeing WDW ever again was out of the question but here we are, going in Sept.!! I just found more creative ways of saving/earning trip money. (These boards were a big help) Sure, it takes a bit longer to save for things like this, but when we get them we all tend to appreciate them that much more. In general, our kids are growing up learning to value things rather than taking them for granted.
So my suggestions for affording a WDW vacation with a family of four would be: Visit the budget board regularly and use the suggestions posted there. My favorites are: selling on e-bay, saving change, clipping coupons/rebates, staying in a rental home(much cheaper than 2 hotel rooms and not so cramped), eating breakfast at home before going to the parks, packing water/snacks, buying souvenirs at home for much less and taking them along.........Oh, there are too many to list.
As far as the overpopulation issue, my brother and his wife have decided not to have any children, so we had the "2" his wife and he would have had.