想買個有線router....

you mean Cable Modem router ?
if so , belkin economical solution should work for you, 4 computers.
http://catalog.belkin.com/IWCatProductPage.process?Product_Id=136521
If you need the commercial one, of course it will be CISCO router ...
This link have all you need
http://www.cisco.com/en/US/products/hw/routers/ps380/prod_models_home.html

有線router已經發展咁耐，各廠的技術已經差不多
普遍市面有售的router都可以
如︰Linksys