I use NetGear GS108 8 port Gigabit Ethernet switches to extend the capabilities of my home network - everything has Gigabit Ethernet coming to it in my house - laptops (when on wired), desktops, even devices that really don't make use of it, like my networked printers....
My setup is basically the same as yours - one of your existing 4 ports connects to a port on the switch an then you'll have some more ports. An 8 port may be more economical in terms of expandability, but it i s up to you to decide.