I'm far from an expert, but my impression is that the adults should take easier to juveniles than other near-adults. The older the new birds are, the more likely they might challenge the established birds, or less likely to follow them.
Let them see but not touch for a week or two, then start letting out pairs of younger birds and see how it goes. If all is well, you can release the other two and they should all stay near the adults. If it goes poorly, the two younger ones shouldn't stray far from the confined ones and you can corral them in and try again later.