Line in (cable or ADSL connection or whatever you have) to modem , then connect the ethernet out from the modem to the 4 port router, then plug your pc's into the router.
With a cross over cable your desktop (if you make it the gateway) will need to be on for the laptop to access the net.
With the router set up as I described you will not need to have the desktop running for the laptop to access the net.