Innocuous "Hang-Up" When Updating

Just as an FYI, FWIW: updating to 4.0 occasions a weird kind of hang-up where, instead of resulting in the familiar “you are now updated screen” (not a quote but a paraphrase, that) I’ve been logged out to the theme’s default 404 page.

I’ve done this now on two different sites of mine running two different themes from two different theme-makers.

BTW, I thought updates were supposed to be fully automatic now — or was that just the most urgent of security updates?? Still had to hit the “update” button as always…just curious.