You don't need to switch them off the chick starter, but you will need to start limiting their access to it. At four weeks, remove feed at night. At 6 weeks, they will only need to be fed two or three times a day. Only offer what they can consume within 20 minutes. And try to keep the feed and water as far from the heat source as possible to encourage them to move around more.
I was going to move to an organic crumble, but they are out until Thursday and I already started taking the food out after 12 hours they don't have access all day though. They tend to knock over the little feed trays I have so they wait until I'm home to refill them, they always have fresh water on the opposite side of light, and I let them out during the day except for the last few days due to cold rain they tend to move around for a little and peck,and scratch for a while before huddling up together