Chickens can get calcium from a lot of different sources. Certain plants and hard-shelled bugs are a possible source. If your native rock is limestone, they’ll get some calcium from the gravel they eat for grit. They’ll get calcium from oyster shell or egg shells. They’ll get calcium from chicken feed, Layer more than the others because it’s got more, but Starter, Grower, and all those others have some in it too. It all passes through their gut and they extract some calcium from it, including the calcium from egg shells.
They don’t digest all of the calcium though. A lot if it passes on through and on out in the poop. And they need some calcium for body maintenance. If their only source of calcium is their own egg shells it won’t be enough. Too much of it gets used for body maintenance or just wasted out the rear.
Let your eggs tell you how it’s going. If the shells are firm enough they are getting enough calcium from some source. What you are doing is working in your unique circumstances. If your egg shells are thin, you need to do something. Some of us offer oyster shell on the side and it lasts forever. They are getting calcium from other sources. For some of us, that oyster shell disappears pretty fast. They don’t have another good source.
If all they eat is Layer, they should be getting enough calcium from that. But if they eat something other than Layer, they may not be getting enough from the Layer.
If all or practically all your egg shells are thin, you have a flock problem and need to address it on a flock-wide basis. If all the eggs are fine except for one hen, then you don’t have a flock problem, you have an individual hen problem. Some hens just have something screwed up with their internal egg laying factory or they don’t have the instincts to seek out calcium. Personally I don’t keep those hens in my flock, but since one of my goals is raising them for meat, my solution is easy. I really don’t have any experience in how you would correct that in an individual hen and I hesitate to overfeed my flock calcium because of one hen. Why put the rest at risk for the sake of one?
That 4-1/2 month old sounds like she has just started to lay. There are several components to a pullet’s internal egg laying factory. Sometimes it just takes a few days to get the kinks worked out. They can lay all kinds of weird eggs when they just start to lay, but most work it out in a week or two at most. In some ways it’s surprising how many actually get it right to start with.