I would consider this much too small for 11 chicks of 4 weeks of age.
Do they still have a heat lamp running?
As indoor chicks there should not be any mites nor lice present.
Hello and welcome to BYC! :frow
How do the droppings of your chick look?
What exactly are you feeding?
Did you already check for lice and poultry mites? And check in the coop as well at night rubbing your finger alog the underside of the roost. In case you keep it/them in the house still: how...