With three very young children, I would say wait, wait, wait! If you still want to adopt in a few years, you have plenty of time to do that, but give your love and attention to the children you have now. I had three daughters and we eventually adopted a son, but my girls were all old enough to be somewhat self-sufficient, and also were very helpful with our son when he came into our family. It's very easy to reach a place of overwhelm when you've taken on too much too quickly, and then you're not able to take care of everyone's needs, especially your own. You're young and have plenty of time to add to your family. Slow down, enjoy your children, take time for yourself on a regular basis, and know that you can always adopt.
One of the main considerations (in my opinion) with regard to adopting is age placement. I've heard it said time and time again that it's not a wise decision to adopt a child who is older than any of the children already in the home. It upsets the birth order and creates chaos in many ways. Allowing your children to get a little older will make it possible for you to bring in a younger child, therefore leaving your family in tact and ready to welcome the new arrival.
In the meantime, explore the feelings you're having about wanting another child so soon. Perhaps you simply want a large family with children who are close in age, and that's perfectly understandable. But if there are other reasons, you should take a good look at that (my rapidly expanding family was all about healing my own lonely, unhappy childhood - I wanted the white picket fence, lots of happy kids, etc., which is the opposite of what I grew up with). The important thing is that the children you already have are getting their needs met and they're thriving. I'm not assuming they're not, I'm just speaking from personal experience when I say that four young children is a lot to handle (as is three) and it's easy to take on more than we are ready for because we love babies...I often found myself in a place of overwhelm with so many children who had so many needs, and although I couldn't even imagine my life without my whole brood, I have thought over the years that it sometimes was too much too soon and I lost myself along the way. Looking back on it, I would have paced myself a little slower in order to give more individual time to each child before adding to our family, but everyone has survived and is happy to be part of our family.
Whatever you decide, and there is no wrong answer here, be sure to take time for yourself. Mom's tend to give it all away and not keep anything for themselves. You will be a happier mother if you have quiet time to do what you want to do that has nothing to do with taking care of someone else. Good luck to you. Adopting can be a beautiful experience for the whole family.
T.