For the past few years, I've used a pi1 with an 802.3af POE splitter.
The splitter outputs an RJ45 ethernet plug, and micro USB with 5VDC @2.4A.
The splitters RJ45 plug only has 2 pair (4 conductors) connected. From left to right, pins 1, 2, 3 and 6 are connected. My understanding is this limits the connection to 100mbit/sec fast ethernet, but that's OK. I don't need speed, just a reliable connection.
Recently, I replaced the pi1 with a pi4 (2Gb). If I connect it to the same POE splitter (using the official usb-c to micro USB adapter) then it powers up, but the lights on the pi4 ethernet socket never light up.
Should the pi4 auto-negotiate a 100mbit/sec fast ethernet connection? Or will it only activate the port with a cable that has all 4 pairs connected?