I use to want 4 also and am having the same problem as you. My 3rd is due in April next year and I too feel I am not giving my other 2 very good mummy quality because I am so damn tired. For me I have decieded that that is a sign that 3 is my new 4 and I am happy with that. I feel I can only cope with 3.

But no need to rush into any big decisions. In a few years you may be thrilled at the prospect of another.
I actually didnt think I would have anymore after #2, my pregnancy was horrific and I told everyone that I could not possibly do that again.
But after a little while I felt back to normal, I exercised, did some things just for me and I felt like me again and my feelings of wanting to have more kids came back and I am having another and I am thinking very differently with this pregnancy alot more positive.

So just give yourself some time is the only thing I can suggest. Dont be too hard on yourself. The fact that you are there for your kids and love them is amazing.
If your plans change from 4 to 3 kids then it does. You will have 3 great kids to love and enjoy.
Good luck