Another new thing.
Previously when two or more notifications came in at the same time for the same app they were consolidated as one notification that just told how many notifications there are for this app and you had to go to the notification center to view them. For different apps they were shown one after another after you handled one.
Now with watchOS4 they are consolidated as well but then are shown right underneath. For the same app they now show the app icon and a badge with the number of notifications and for different apps they show a neutral icon as well with the badge.
I couldn't screenshot this behaviour yet as it is way too fast, but it's a really nice and welcome change.