My Forerunner 645 ( not the Music version) has been automatically updated overnight to 4.00 2 days ago. An annoying issue emerged after only a few hours of use.
If you listen to music and you use the music widget to control the volume or play/pause/skip a track that plays on your phone for example, it will cause the watch to crash and reboot...I hope Garmin will fix this soon...it’s embarrassing that such a large bug was not found.