the weird thing is is that I just noticed it when they were swimming today and then when they dried off is gone
X2I still think they are not 4 weeks old. The pics you posted when you got them last week look like they were around a week old. Here's what pekins look like at 3 weeks They are just going to need some heat, especially after getting out of the water. If they are getting that soaked when swimming, I'd start swimming them in warm water more often for less periods of time to encourage them to preen more and get those oils distributed over their body.