They seem fine to me. We have 5 ducklings also just shy of 5 weeks old and they look very similar.
If they're getting exercise (looks like they are) and you have them outside where they can forage (it looks like you do), I don't see that they would get it. At this age, we like to supplement...
We lost a duck last week, meaning that we had 3 drakes for just 1 hen; horrible dynamic.
I found a guy on Craigslist who sold me 3 more hens, all mixed. I picked them last night, and this morning discovered the 4 hens had laid 6 eggs among them :-)
I'd love to figure out what types of ducks...