THIS STILL REQUIRES MI FIT AND YOUR EXTRACTED KEY
HOWTO:
1) press + button in Gadgerbridge
2) LONG PRESS Mi Band 4
3) Tap "Auth Key"
4) Enter your key prefixed with 0x (eg. 0x112233445566778899aabbccddeeff00)
5) Go back
6) Tap Mi Band 4
Success? You tell me.