Agreed with @nuthatched and @rosemarythyme above. I'll just add my 2-cents, since I started this reply before they posted. (They're just much faster and less wordy than I am!)
My chicks are about a week older than yours.. The starter I bought is medicated. That feed will run out at about 8 weeks. This feed is in the brooder where the hens can not get to it. At 8 weeks I will switch the hens and chicks to a non-medicated grower. There will be a side dish of oyster shell. In the Fall, the chicks will start laying, while the hens start molting and stop laying. At that point I will switch them all to Game bird pellets for the winter. Lastly in the spring, when most of the girls are laying they will move to layer pellets. This schedule has worked for us for our 6-7 years. I like pellets as there seems to be no waste.
4 weeks should be enough but there's no reason to waste the unfinished bag if you have more left. Feed it to them until it runs out, then get unmedicated for the next bag. If you have older chickens, they can eat chick starter as well, both medicated and non medicated, it won't hurt them at all and their eggs are safe to eat. So don't feel like you have to separate them or separate who eats what, or buy multiple bags - you can just feed them all the starter (with eggshells/oyster on the side if you also have layers).
Answer actually depends on where they're located. If they're already outside and have been for a week or two, then you can probably switch off whenever. If they have not been outside at all, then it'd be safest to keep them on medicated feed for a couple of weeks after they move out, as they have not been exposed to any coccidiosis in your environment and it seems like the transition period is when they're most likely to have problems.
