Good morning!
As you might have noticed, we've upgraded to 4.0.1. Please give @hugo hugs, he was just going to bed at 5am after upgrading everyone on the instances he hosts :/
One of the main reasons people were excited for the 4.0 release is for the 'edit' function so that they could add missing alt text.
Surprise! You can't add missing alt text with the edit function. ? ? A work-around is to remove the image and re-add it, which will get you the alt text field back on the image.