Can you post photos of all of them? It would be interesting to be able to see them all...
Also would be interesting if you let us know what response you get from the breeder.
I wonder if they'll feather up eventually or if they'll always have cold bums?
Also their fluff in general seems to be quite sparse. That bottom photo, you can clearly see the skin on the little one's chest through his fluff. I'm assuming that lump is his crop. There should be LOADS more fluff than that on him. As far as I know Light Sussex in general don't tend to feather...
I don't know about the baldy bottoms but just looking at your pics I'd say that for their age, their feather development in general seems very slow. Are the ones with normal fluffy bums feathering out at the same rate? By four weeks my chicks usually have much more feathers than that...