I've had a broody hen that let her chicks go at 4 weeks; it was painful to watch because the chicks were not ready to let go yet and chased their momma around when they could find her. The two little ones seemed so lost without her. I would let them out in the morning and they would trek down to where she took them to forage all by themselves. I thought that was too early and didn't use her again as a broody.
I also had one broody that brooded her chicks until they were about 14 weeks. I thought that was a bit long, but the chicks didn't seem to mind the helicopter parenting. It looked really strange for that mother to have her wings spread out over such big chickens! Obviously, that is a little bit of a waste of a good broody because she could have been working on another group of chicks instead.
I think each broody is different because their hormones are different. It's prolactin that keeps them brooding chicks, I believe. So it's not like a broody can control how she acts. But I try to keep track of which broodies do best and use them again.
Interesting part ... there comes a time that chicks stop making chick peeps. I think it's around 6 weeks. To me, that makes more sense naturally as a time to "kick the chicks out of the nest" ... or at least not before then. 8-10 weeks seems to work well also.
If your newest broody is happy with the 4 week old chick hanging around, hooray. I think possibly the fact that she is still making chick peeps makes a difference ... I'm not sure, but possibly.