That's pretty normal behavior for chicks getting enough feathers on their wings to achieve lift-off. As long as the days are mild, around 75F, you could let them go outside in a playpen for some exercise. Give them a protected area ten feet long and watch them achieve low-level flight!
Then bring them back inside when it cools off. I usually carry my chicks outside in a pet crate, then leave it for them to nap in. If you see them huddling in the crate more often than fplaying outside, it's time t bring them back inside.
I usually move my chicks into the coop around five or six weeks.