I'm thinking going by my own that hatched here 10 weeks ago, that at 4 weeks they are still going to be about the same size. Usually around 6 weeks you start to see the draklings out growing the ducklets. and then it begins to be obvious, But try it and see if you notice much difference I'd like to know.
I have 2 ducks and 1 drake 10 weeks old my one duckling from hatch has always been larger than her sister, I felt for sure she was a he, so sometimes you cannot go by size YET. But I have noticed other tell tale things too, from previous hatches over the years also, my drakes at about 5-6 weeks old will lay down a lot, they aren't near as active as their sisters are. This will go on for about 2-3 weeks then they seem to get their legs under them and don't do it as much. I don't know if it's because they are growing so fast but I have had duckling hatch the last 3 years and it was this way on all my drakelings. Other ways to tell duck from drake. legs drakes will be thick and set farther apart under body, ducks will have thinner legs and almost step on their own feet when walking the ducks bodies will be shorter and rounder than the drakes he will be longer from tip of bill to tip of tail. most of my females just have a feminine face.
