Integrating newbies into small flocks can be difficult. When your young birds are about 4 weeks old you would be very wise to fence in an area for them, or fence off part of the run, where they can spend their days next to the older birds but protected by the fence. If you just pop these young birds in with the older ones when they are only 4 weeks old you could easily have dead youngsters. Chickens HATE newcomers and they can be brutal. I don't integrate new young birds until they are the same size as the older ones, 10 to 12 weeks old, so they can better protect themselves. They move out to their grow-out pen next to the older birds at 5 weeks old but they don't go out to range with the older ones until 10-12 weeks. By that time everybody has had time to settle down and get used to each other and pecking order scuffles are very minimal. The slower you take integration the easier and less stressful it is on everybody, including you!