Only 4 wires are used for 100mbit ethernet. However, all 8 are used with 1000mbit (gigabit), and that is the speed he needs between his cable modem and router if he wants to use his full 120mbit internet bandwidth.

His current router, and the ones he has looked at as a replacement all offer Gigabit WAN ports. Some places in the world have decided to keep building faster intertubes for all the big trucks