Nicholas,

Hi. Routers in general have firmware you can access. This contains a page or two of info wherein you can view who is logged in at the moment.

If you want to make sure only your 4 PCs and the Apple Mac are allowed, you can, use the MAC Filter feature usually present in routers.

If you really want to be more serious and monitor whatever one PC does, you can setup one as a Firewall Gateway.

Happy Holidays!

Dennis