8 week old chicks are fully feathered at that stage. They should get along running around your coop, and run, just fine. Feed them a non layer feed until they start laying eggs. Non layer feed contains 2% calcium, while Layer feed is at 4% calcium. There are many options to feed your flock. I can expand on that if you need more info there. Otherwise, my posts would get way too long with info that may not be needed

You will have to integrate them with your 2 older hens. The older hens are obviously bigger, and stronger,,,,,,, that translates to them being possibly aggressive to newcomers. If you can house them together, with a separation of chicken wire,,, to see, but not touch, they do get used to each other.
No idea how your housing, coop and run looks. So cant offer much more what I don't have a background info on.
Other things missing,, is your location in your profile. No way to give advice on weather/climate related issues since you may be anywhere on the planet earth.
We don't need or want your address,,,,, just a general idea like City,,,, or State,, or country.
WISHING YOU BEST,,,,,,,,,,,,,,,,,,,,,,,,,,,,and