I've bumped my 4 against multiple objects without any resultant scratches. My 3 has a deep scratch near the top from brushing a wall.Finally got my Mi Band 4 from Banggood and the screen picked up 2 scratches on the 2nd day of using it
Seems to be a common "issue" from what I've read on Reddit so order a screen protector for it if you plan on buying one.
Had my Mi Band 2 for 2 years and it didn't get a single scratch.
The problem with a screen protector is you won't be able to swim/bath/shower with it on anymore.