At four months old you should be able to tell the difference in the quack...the boys are raspy, almost like a loud duck whisper. The girls actually quack. I could tell my Indian runner drake (that was supposed to be a hen) was a drake at 2 months due to his voice. If your duck quacks loudly, chances are she is a she.
Now they're 3.5 months and the drake has his beautiful green sheen (he's a black) and his curly tail feather.
Since I have three hens, I don't mind. The boys are quieter and it just means I could incubate duck eggs if I choose to.