I've done this and it's fine. I'd never risk mixing very young chicks in with bigger ones, even if there was only days of difference, there's too much chance that the young will get deliberately trampled.
but later on it's fine. 7 week and 3 week olds should be fine, especially in such small numbers.
Someone on here said that they always mixed batches hatched 3 weeks apart so i started doing the same; refilling incubator as soon as first hatch had finished.
I'd recommend that you're around if possible for mixing day and check them frequently. If the hutch is small, the big ones will generate heat for the younger chicks as well, so as long as the weather is fair, the hutch smallish, and the run weather proof, you can probably take them off heat sooner than you might if you were just putting the four wee chicks out. Mum would have her 3/4 week old chicks out most of the time in good weather. I like a roof for my chick runs and shelter from wind. In a bad spring I've had some in the greenhouse, other years put old windows at sides of run and a tarp over them.
It also depends on the size of the run, if it's bigger, with several water and food stations, places to hide, jump on and stuff to distract them, there's less likely to be problems. Sorry this is a bit long and ranty!