I believe it has to something to do with parallel transmission. 8 wires equals 8 bits ot transmision, or a "byte" at a time. 4 wires equals 4 bits, a "nibble".
You are correct for parallel communications, but Ethernet TCP/IP is serial.
Technically only 2 conductors are required for Ethernet, but the common Ethernet TCP/IP uses 4 conductors.
Ethernet is color blind. It does not care what colors the conductors are; only which pins they are connected to.
Very briefly and not technical:
Even today, most Ethernet communications (end connections to laptop and desktop PCs) involve speeds in the 10M class. Because it is 'slow', this speed is very noise tolerant, so pair twisting is not an issue and actually only requires Cat 3 cabling. Connections to routers and switches and most servers involve speeds in the 100M class. This speed requires a more controlled twisting of the conductor pairs so Cat 5, and its enhancements, cable was created. The pairs of conductors in these cables have a different number of twists, so placing the correct color conductor on the correct pin will allow the most performance out of the system Devices that push lots of data, really fast, like real live server rooms need speeds in the 1G+ range. Here the geometry of the conductor pair spacing as well their twists is very critical as is color conductor to pin matching and the length of 'untwisted' conductors.