i will have this same problem since i have 1 week old chicks now and my eggs are due to hatch in 3 weeks. from what i've heard, 4 weeks is too long of a gap to put the day olds in with them. you have to wait till they are more of the same size. what i'm going to do (and this depends on where you live) but my older chicks will be outside in their own separate coop and run (separate from my adult chickens). then my brooder will be free for the new guys. then once the new guys get to be about 3 weeks old, out they go with the older chicks. and i'm going to introduce them all at night when the bigger chicks are sleeping. i read that's a better way to introduce new chickens.