I have two sets of chicks. Seven are 4 weeks old and recently have been introduced to outdoors and have a heated box at night. Three are two weeks old and still indoors. The smallest of the seven seems to be lowest in pecking order and often hangs back. Would it be okay to put her in with the other three chicks?
Personally I would not. My goal with integration is that no one gets hurt. As long as that chick is not being hurt and it is doing OK where it is I don't see any benefit in putting it with the three young ones. Sometimes the lowest in the pecking order can be the most brutal to younger ones if it gets a chance. If things are going OK don't fix it. You might break it.
I don't know what behaviors you are seeing. You may see something that looks threatening to you where putting that chick with the young ones is the right call. You are looking at them, trust your judgement.
Thank you. Do they still need to be fully feathered when in 75-80 weather outside?
Not at four weeks of age. Many are actually fully feathered at 4 weeks. Certainly by 5 weeks.
I brood outside. One ridiculously hot summer I turned the daytime heat off at 2 days and the nighttime heat off at 5 days. I should have done it earlier. Their body language said they did not need it and they did not. I don't remember the nighttime lows but the daytime temps were around 115 F. You need to react to your conditions.
A couple of times I've had a broody hen wean her chicks at 3 weeks of age in temperatures close to what you are seeing. Those chicks were left totally on their own to make their way with the flock and handle staying warm on their own. They did. I don't know what kind of nighttime temperatures yours are seeing but you have heat available for them if they need it. My three week old chicks did not.
It is a long story but one time I saw a chick get separated from a broody hen when it was under one week old. The daytime temperatures were probably around 80 F, not any higher. That chick spent all day by itself, did not get back with the broody hen until pretty late that day. It complained a lot during the day because it was by itself but it wasn't hurt.
When you trust your broody hens to take care of their chicks you learn a lot about how tough those chicks can be. I'm not a broody hen so when I brood them myself I've more careful. I would not turn 3 week old chicks loose to make their way with the flock. I would not leave a 1 week old chick away from heat in those temperatures. But I'm not a slave to some of the numbers I see on here either. You can learn a lot by observing your chickens.