At 4 months you should be able to sex them by size.
So chances are you are correct with that. The large ones are males, small ones are females.
White is very common in colored muscovies. In fact, it is hard to find solid colored ones that have zero white on them! So yes, they do look like pure muscovy ducks and some may develop more white as they age.
As for the colors, the dark brown are chocolate. The lighter one could be one of a few things.
1. It could be a chocolate that is diluted from the sun and ready to molt. But if they are all the same age, and 4 months old, this should not be the case just yet
2. It could be a buff-chocolate. There is a lighter colored chocolate gene that some carry and this could be the case.
3. It's a blue fawn. If the feathers (it's easiest to tell right after a molt) have a blue hue to them, then it's a color called blue fawn. They can easily be mistaken for a chocolate if you don't have chocolates to compare with, or easily mistaken for a sun-weathered chocolate that has faded color. The only difference is they blue/gray color mixed into their feathers. But as they get sun-weathered, they can look identical to a chocolate duck