DO NOT USE this old experimental code. For usable Ethernet on Teensy 4.1, use the QNEthernet or NativeEthernet libraries which provide a complete Ethernet networking library.
https://github.com/ssilverman/QNEthernet
https://github.com/vjmuzik/NativeEthernet
This old code was only the very early experimental code used to first test the Ethernet hardware and begin development of a low-level driver.