@konczakp In 10/100 Mbit/s networks only 2 pairs are used (one for RX, one for TX). Other two can be used for other things (e.g. PoE).
I think that using Ethernet is easier then RS485. Whore hardware part is done by NIC (Network Interface Controller), you just need to send/receive data. There are STM32 MCUs with Ehternet hardware built-in (e.g. F107, F207, F217, and more) or you can use external NIC that communicates with MCU using UART (e.g. CH9121) or SPI (e.g W5100, ENC28J60).
There are also libraries for Ethernet for Arduino board and STM32duino.
There's a topic about using Eth on STM32 (different matter, but show how to use Eth to send UDP packets):
viewtopic.php?t=1732
And using ethernet you can easily connect PC to the network and monitor all traffic (using network sniffer program like Wireshark).