Hey,
I'm not an expert on adoption, but it would be worth checking out
Welcome to Travel.State.Gov and clicking on the specific countries adoption policies, which can change from day to day. For example, Vietnam is not letting ANY Americans adopt from there for the time being and China has a stricter policy on adopting (for example, being 30+, married for a few years, etc.). Like I said though, these policies can change at a moment's notice, so it will be wise to keep up with them every once in a while.
And, reality check: 4-year-olds can be just as terrible as babies LOL! They still puke from time to time, get sick, have "bathroom accidents," whine, cry, just all the lovely things that makes you so glad you have them! AND they can walk and talk you death too LOL! Having kids can be a daunting and tiring task, one that surely doesn't get easier over time! Also ask yourself if you are willing to adopt a special needs child as well or how you will cope if it comes out that your child has some type of developmental delay.
It will be good to hang around people who have children and to get information from an international adoption group in your country that can help you with adoption resources. I'm happy for your decision and I wish you the best of luck. Happy research!