I have not had any reports, warnings, violations. I haven’t left/disconnected any games early. I’ve played for 5+ hours today so it’s not decay. I suddenly got a message that says I went from 4 to 3 for no reason.
According to Jeff you should keep getting endorsements to stay at your level(Developer Update | Upcoming Social Features | Overwatch - YouTube). The higher the level, the more endorsements you will need. Can it be that you just didn’t get any endorsements for the past X hours?
I don’t think that the mechanism/logic is known to anyone except for Blizzard, so my random guess is that you didn’t qualify to stay in level 4.
Yeap, got the same issue on weekend. Out of the blue went from level 4 to level 3 while playing and receiving further endorsements.