If your chicks are only 4 weeks, they haven't really gotten to their adult plumage. They will molt several times before reaching adult plumage. In the early stages, they may have buff on the tips and markings that look like lacing, but as they mature, their feathers should become solid red. If that lacing is still there when they are mature, then yes, they could be crossed with something else in that line that produces some odd markings. Big hatchery stock is often not right on the money for color or type either, most noticeably the size and weight of the birds.