Agreed. By far the easiest thing to do is replace the router with a Wifi 4 port router.
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“The Government of Spain will not applaud those who set the world on fire just because they show up with a bucket.” - Prime Minister of Spain, Pedro Sánchez
Correct. All your existing computers can stay wired the way they are, you're just swapping out a 4-port router for a WiFi 4 port router.
I would recommend using a USB WiFi NIC for a desktop, and a PCMCIA WiFi NIC for a lappy. The USB works better than an internal slot based NIC. You can get the unit high up on a desk or shelf for better reception.
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“The Government of Spain will not applaud those who set the world on fire just because they show up with a bucket.” - Prime Minister of Spain, Pedro Sánchez