H.A.
No one can answer this for you.
You need to know what your family is capable of handling. Size of family is very much dependent on the family and what they want out of life. Large families means some sacrifices normally -- less vacations, more expenses (doctor, food, clothes, activities), travel is harder with a big brood... even "bargain" plane ticket prices means dropping 1-1.5k when you're flying 6 people. There are odd things to consider - like how hard it is to fit 4 carseats in cars that supposedly seat 6-7 "people".
Besides the practical adjustments to consider -- can you afford it? Are you self sufficient right now (no government support - healthcare/food/etc)? How much do you want to be able to help your kids -- first car? College? wedding? How would one more affect what you can do for your other three?
Lastly, do you want one more? This is the most important. If you have to ask a bunch of strangers... how bad do you really want another child? I just knew we weren't done, where I have friends that knew they WERE - be it one child or six... there is a comfort spot where your family will feel complete. Don't confuse baby fever with actually wanting another person in your family. It's like thinking puppies are adorable but not really wanting another dog.. I know I will miss being pregnant and nursing my newborn, but that doesn't mean our family needs to expand more. Make sure you can tell the difference in the pangs you're feeling.
If I were you -- I would be having this discussion with my husband, not a bunch of women on the internet. Maybe he can help you and chime in what is best for your family.
Good luck, whatever you two decide.