I’ve practiced the 4 week dark system for the last 3 years. Here’s what I’ve found. I put them on dark on the first weekend in May. By this time my early April weaned young birds have started the primary flight moult. During the 4 weeks of dark the head, neck, body and small wing feathers will all be replaced. The primary flight moult continues but slows down. After the 4 weeks of dark I just leave them with natural light. The main drawback I’ve found is that the cover feathers don’t moult during the 4 weeks. They start to get cast them early August. So you have to watch for wet days. The continued moulting of the primaries is not an issue for me as they seldom throw past the 7th flight by the end of the Racing.