Are you sure the egg she is sitting on is either not fertile or has stopped developing?  You may want to candle it to be sure.  If the egg is not viable, I think your best options would be:
 
1. Take the egg and break her of being broody
2. Take the egg and chance putting other fertile eggs under her to see if she will sit another 3 weeks (provided she is still in good condition having already set 4 weeks)
3. or Find chicks that are only a day or two old to put under her (In my opinion week old chicks are too old to try this and it could end very badly)
 
Either way, I would recommend separating them from the other birds.  Even if your silkie has already proven herself to be a very protective mom, you will likely want to wait until a bit before the chicks are allowed around other birds. Roos and other hens have been known to go after another broody's chicks.