The earlier you integrate them, the better. However, day olds are not really going to do well at all with 4 week olds - so have them separate until, oh say, maybe three to four weeks of age (the older ones 7 weeks). Make sure they spend some time together looking at each other through some fencing, if possible. This is just what I have done, and you would always need to supervise for quite some time (I usually spend TWO HOURS watching!). If you see anyone being attacked viciously, separate them again.
You might try again in one week and see how it goes, if the first attempt at integration fails.
At three weeks of age, the youngsters really start chest bumping and becoming more trustworthy (although I do check on them frequently) about going in and out of the pop door to their light bulb (this is the age at which I let them venture out of the coop to a small fenced area with total access to the pop door/light bulb inside the coop) and they are able to interact with other chicks who are a bit older from my experience. But chickens are individuals and you must see how yours do/see how you feel about it at the time.
The earlier the better, since if you want them to love each other in the short term, they need to grow up together. Otherwise they will be two groups that don't really like each other until a long time passes from my experience.
