I think you meant "switch"
Of course the simplest thing to do would be to buy a 4 port router and just add a switch, as theasian100 said...because most general computer places only sell consumer level routers and probably won't have any 8 port routers. There wouldn't be a performance difference between a 4 or 8 port router, if it is a 10/100 router then you will get that speed on all ports, whether you have 4 or 8.
Most people these days don't even have enough traffic or clients on their consumer level routers to reach the maximum throughput. The average Joe goes to the store, gets lured into buying a $100 router he doesn't need, and then surfs the internet and does e-mail, both of which are fairly light tasks and will put no strain on the router whatsoever.
And YES to clearly answer your question there are routers with 8 ports...there are also enterprise level routers with many more than 8 ports.