I've been running 3.8.0 for some time, which has mostly worked OK. However, yesterday, the Venu started drawing a lot of battery, and the charge level quickly went down. I decided to give 4.0.0 a try.
First, I tried to start the update on the Venu itself. When entering the Software Update menu, all I got was a blue moving circle. Stopped after a minute or so, but just returned to the previous menu. Tried several times.
Then, I tried Garmin Express. I was unstable as well: Sometimes claiming that I was already running 4.0.0, sometimes suggesting an update from 3.8.0. In any case, after trying a few times, I was able to initiate the update.
Unfortunately, the Venu update progress is now stuck at about 50%! I actually had this problem when upgrading from 3.5.0 to 3.8.0 as well. I was able to solve it by plugging in the Venu to a PC running Garmin Express, but this does not work now.
At least one other person is having the same problem: https://www.reddit.com/r/Garmin/comments/efqsq1/venu_stuck_in_software_update_i_cant_get_it_to/
To be honest, this is beyond bad. If nothing else, at least the firmware update code should be stable, but apparently it is unstable in both 3.5.0 and 3.8.0.