If I understand what you're asking, you want to know if you can buy an older Silkie than 4 wks, maybe a year or older, and put it in with your 4 wk olds. You aren't real specific about what you're actually wanting to do. If you want to buy a new Silkie and throw it into the pen with your 4 wk olds immediately, I would be reluctant to do that. There's a possibility the older bird will harm the youngsters, and the youngster will likely be afraid of the older bird. The best way is to introduce them to each other over a period of weeks with wire between them. I am basing my answer on other breeds of chickens. Silkies may be mellow enough you may not need much introduction, but I would error on the side of caution.