The integration with SAA was flawless with MB3, but there must be something different with the 4 because it is not sending HR info to SAA.
I left mi fit installed as advised and disabled sleep assistant.
I'm seeing the same.
I got my Mi Band 4 last week and it works fine with Sleep as Android. However the built-in N&F sleep analysis is FAR less detailed in its heartbeat reporting than sleep for android even though they should both be using the same data. I configured the sleep HB detection to be every 1 minute and yet the sleep report shows a useless almost flat HB graph with datapoints being as much as 17 minutes apart regularly.