they very well could be 4 drakes. The darkest could just be younger, in a molt or the drake curl not grown in yet. i say could be all drakes because even the darkest has a pretty distinct green sheen showing on its head. They all appear to be barn yard mixes, likely from a flock of mixed bred ducks allowed to breed. Since domestic ducks are all derived from mallards when you breed mixes you can get all sorts of colors popping out and the green sheen seems to dominate when mixes are allowed.
if the darkest (first picture front right) is a duck then you need to get more ducks or separate two of the drakes into their own yard so she is left with just 1 drake.