Install custom Watch Faces
Xiaomi MI BAND 4
1) Download any .bin file for Mi Band 4
2) Now you have to choose one of the watch faces you don't like from Mi Fit (you hate all of them, probably)
3) Sync that watch face you don't like to your Mi Band 4 (After this, a folder containing the .bin file will be created)
4) Open your phone File Explorer and go to: Internal Storage-Android-data-com.xiaomi.hm.health-files-watch_skin_local (there, there will be as many folders as watch faces you downloaded from Mi fit, these folders have a rare name like "HGDhgsdKGHGDSJHASDHJHKJHDSO" these folders contain a preview image of the watch face (this way you can identify which is which), a .bin file, and an xml.
5) Now you have to move the .bin file of the custom watch face you downloaded to one of the Mi Fit watch faces folders mentioned in step 4, and rename and replace the old file with the new one (so you have to rename the new custom watch face with the same name of the old one, so we are replacing it). This way we're making Mi Fit believe that the watch face we downloaded is the Mi Fit downloaded one.
6) Now, in the Band Displays section on Mi Fit, select the Watch Face we replaced (it has the same preview picture as before) and press sync now, the new custom watch face will be applied!