In my experience, the most important thing is for any smaller chicks to have somewhere to escape to and can run away and that the bigger chicks don't bother pursuing them very far. For my setup, I have 2 regular sized pop doors, one on either side of my coop. When the big chicks come into the coop through either pop door, the little ones generally exit the coop out the other one. I free range (truly free range all day, zero fencing, with a predator-proof coop I lock them in at night) so the 2 groups of chicks can go where ever they want. The smaller chicks just go to places where the big chicks are not. Both sets of chicks will use the coop during the day, sort of trading it off, stopping in for short periods of time to get some chick feed and just hang out for a little while. Then, they go back outside and into the woods or to a cool shady part of the rest of the barn. At night, they all share the coop and the roost and I try to be available to supervise the bickering on the roost. If it gets too rough for the smaller chicks, I might step in and re-position a big chick away from them but 90% of the time, they work it out among themselves.
However, yours are very different in ages, 4 weeks vs. what I think you said is the rest of your flock which is full grown. In this case, I personally wouldn't risk the big ones accidentally getting to the little ones until the little ones are nearly the same size as the rest of the existing flock. With any slot or opening, I have a few of the older chicks that could squeeze through a hole or slat that the others couldn't, just because the breeds are different weights/heaviness.
Mine are only 2-3 weeks apart in age. I integrated them by placing the smaller ones inside the coop in their own enclosure for about a week. Then, I let them all out to free range after the week was over. I left up the fencing inside the coop but had it open so the little ones could still use the space to escape, which they did. The big ones would go in there too, but not that often and when they did, the little ones just left. I put in a picnic table type bench in their little fenced off area of the coop for the little ones to use as a roost, which they did while they were unable to get out of the fenced area and then also for the first few weeks after they were "integrated". And even now, when things get too rough, a few of the smaller chicks revert back to roosting on the picnic table bench for a night or two before trying to push their way into the roost again. After about a week of them being "integrated", I took the fencing down and just left the bench. I eventually took out the little chick's extra food and water too because again, they trade off using the coop during the day.
In any case, just sharing what I've done and experienced. I do really like my 2 regular size pop door set up as it gives the smaller/less aggressive/lower on the pecking order chicks more ways to get away from the others.
Guppy