You have different options. The one I use is to feed them all the same feed, Grower in my case, and offer oyster shell on the side. That way the ones that need the calcium for their shells normally eat it and the ones that don’t need it don’t eat enough to harm themselves.
Can four month olds eat Layer? I don’t know. There are plenty of studies that show feeding excess calcium to growing chicks can harm them internally. But I’m not aware of any studies that show when you can start feeding them the excess calcium. There are those on this forum that feel you should never feed Layer to any non-laying chicken, like roosters or pullets/hens not laying. Others regularly feed their adult flocks Layer even with roosters present. A normal recommendation is to start Layer at 18 weeks or when you see the first egg.
It’s not how much calcium is in one bite, it’s how much total volume of calcium they eat in a day, plus how many days in a row they eat that volume plays a part. If yours free range or forage for a good part of their food or you feed a boatload of treats they may not be getting that much total calcium even with Layer.
There is little consensus on this forum as to what the right answer is. Everyone has an opinion. My opinion is that if I feed Grower (or Starter if I have baby chicks) with oyster shell on the side then I am doing the best I can.
