I think obviously MI band 6 has much more features but it also on our personal need that we want it or not. For example I have a MI band 4 and it has all the features I want and if I buy a MI band 6 then also I would use those features only which I use now in my MI band 4.That was my opinion
I think that if you have the money, that the mi band 6 is worth the price increase over the miband 4. One thing that I miss about the Miband4 since moving to the Miband 6 (I didn't use a miband 5) is the battery life. With everything maxed out (screen on time, screen on with wrist movement for 16 hours a day, screen brightness, heart rate recording every minute) on the Mi band 4 I still got about 10 days with a 2 year+ old battery.
I struggle to get 5 or 6 days with the Miband 6 and have more features turned off. I also am not completely sold on having no on screen button to go back. But I will learn to use the swipe method until it is second nature, I am sure.
The benefits for me on the Miband 6 are many. I really enjoy the improved tracking of sports. I can review my sports history on the band. The map tracking on the band for the path I took on my walks, runs and cycling is great.
I also really like that when a sport is being recorded and I need to stop, then my sport recording is paused until I start exercising again. This is terrific and gives me a much more accurate result.
Elevation is now recorded on the MiFit app via the Miband 6. That data is also transferred to Strava app.
The increased screen size makes reading the Miband6 much easier for me. Without glasses I was struggling to read the homescreen at times on the Miband 4 even though I had a large 3rd party homescreen.
I also like the REM sleep, Breathing Quality and PAI info that is now available to me which was missing from the Miband 4.
So for me, I could not imagine going back to the Mi Band 4, but if you didn't know any different, the MI Band 4 would still be a great unit for what it is.