So since today we have a working TWRP for the Xiaomi Mix 4!
https://forum.xda-developers.com/t/recovery-unofficial-twrp-3-5-2_11-odin-mi-mix-4.4341271/
Now that we have a working TWRP for the Mix4 is it possible to make a zip of the ROM instead of using fastboot?
https://forum.xda-developers.com/t/recovery-unofficial-twrp-3-5-2_11-odin-mi-mix-4.4341271/
You have to flash TWRP (Polaris) within fastboot. After that place the Xiaomi.eu rom on the phone and flash it within TWRP. You will have to wipe data etc if im right. You can see the instructions in the official thread of the latest weekly build