OP, if this is real then please come back and answer previously asked questions. For example, do you really think your parents would be thrilled if your 16 year old sister had a child, and why? What sort of financial position are you and your fiancé in? (Children are very expensive and get more expensive as they get older, especially if they have special needs). Most importantly, why - when you have three young children already - do you want another?
Personally I agree with your parents that you having another child would be a terrible idea, although as you are a grown woman and they don't support you it's actually none of their business.
But, you are engaged so therefore planning to marry. Why not do that first? It would give you and your children a lot more security. (I only say this because as you are engaged, presumably you are both on board with the idea of getting married).
I also agree with previous posters about that fact that it's selfish not to consider the environmental impacts of a large family. You already have three children. There is absolutely no reason to have more. (Plus, as you well know, it wouldn't necessarily be just one more.)
Given that you are so young (is your fiancé a similar age?), if you stop having children now, you will still be young once they are grown and will have lots of time left to enjoy your older years. Assuming your children have no additional needs, by the time you are 50 your kids will all be out on their own and independent. What a lovely position to be in!
I can understand your annoyance with your parents, but please think carefully about whether another child/ren is a good idea and whatever you do don't have one just to prove to them that it's none of their business!