It sounds like there's only one four week old- is this accurate? And I've heard you can mix chicks with fewer difficulties if they're within 3 weeks of the same age
I asked if it was a single chick because that makes it a touch easier. We raised a single chick, Calliopeep, alone until she was six weeks. Then we ended up getting just one more chick, Sesame. We let them spend a night where they could see and hear each other but not interact. The next morning, we wanted to see if they could meet. We watched closely, but they got along without a hitch. Callie appreciates the company, and Sesame thinks of her as almost her mom. The size difference is so extreme that it's adorable to see them together. Anyway, that probably won't work for you, but single chicks seem to inter grate better with younger chicks.
(Here's a poor quality picture of Callie and Sesame