I'll tell you what I've done.  I don't know if it was the best way, but nobody is bleeding or dead and they were 4 weeks old.
We have three ages: 14 weeks, 10 weeks, and 5 weeks.  They are all living together now, but the 5 week olds aren't really integrated yet.
With the older two groups, I basically just added them without any prepping to get them used to each other.  They were both new to me and the now-10 week-olds outnumbered the now-14 week olds so there was not a lot of violence. 
When I sent the babies to live outside last weekend (at 4 weeks), first I had them in a dog kennel for a few hours a day outside so the older ones saw them then.  The older ones also saw them when I let them explore in the yard for about an hour a day.  That was under supervision and there were a few times I had to step in as mother hen to protect them. 
Then when I finally moved them outside to live over the weekend, I placed a cardboard box in the run with holes big enough for the babies, but not big enough for the older ones so the babies could run for safety if necessary.  The first few days, the birds were free ranging a lot so the babies could pretty much stay clear of the bigger ones if the bigger ones were trying to peck them.   
I figured that that is how it works in nature and nature isn't wrong.  Just make sure the smaller ones have a safe place to hide for a while until the older ones get used to them.