One of the values of introducing them at 4 weeks is the older birds don't think of the younger ones as much of a threat. Not like if you bring them in as adults. Most of the "fussing" will be over food so hopefully the littles will figure out they have to let the bigs eat first if they are being aggressive. Eventually there will be more fussing as all the birds figure out where they want to be in the pecking order (which isn't necessarily where they will end up).