Depends a bit on the birds and the age of the youngest. 4 weeks apart when they are 14 weeks and 10 weeks isn't that big of a deal, but day olds and 4 week olds is.
Since you are hatching sucessively, I say about 3 weeks is all you want to do, since they will have a different temp requirement. Just let the newborns have their own brooder for a few days or so so they can get up onto their feet, before popping them in with the older ones. Keep the range within a month though as some chicks chould easily smush the younger ones.
However, I have done up to 4 weeks difference, but the oldest one was just a lone chick so was happy and just fine having a bunch of week olds running around.