I was able to follow the instructions on [solved] Intel i226 Firmware (see post #39) - Page 8 to flash a new 2 megabyte firmware image to the onboard NIC.
This resolved an issue I had with the factory (2.17?) firmware and the NIC being generally unstable. Previously under Linux it would only link at 100 megabit and drop out frequently.
Interestingly, under Windows it was stable at 1 gigabit (I haven’t tested 2.5) with both firmwares.
Radxa, you should ask Intel for the newest firmware image for the chip/s you’re selling.
PS: Will there be any further BIOS revisions for the X4 without eMMC?