At only four weeks old there really sure answer by color. Any green that you see on the ducklings heads doesn't mean it is a male. I know when my Rouens were babies I thought for sure that's what it meant but it turned out that even my girls had green on there heads until they were older. In another week or maybe even now you should start to hear some changes in their voices. They females will start to quack while the males will be harder to tell and might not start to make noise for a little bit longer. They have a raspy quack almost like a wack instead. Males do have green heads but that won't be until about 10 weeks or so that you will be able to tell the males from females by coloring.
Thanks!!!