My baby chicks are about four weeks old and we moved them into a pen in the chicken coop that has three older chickens a week ago, and I don't know if I can take them out of the little pen into the actual chicken coop because the big chickens can fly in there already so can I let them roam around the chicken coop yet or not.

and Welcome To BYC! Good luck with your baby chicks! TwoCrows gave you some good advice, it is best to wait until the chicks are about the same size and then introduce them gradually. There is a nice article in the Learning Center on combining flocks, with different ways of doing so (after the quarantine section).