YouTube is one of Google's most server-side test prone apps. Every couple of weeks, sometimes even days, we spot a new layout somewhere that brings a little change to the table. The latest is a renamed 4th bottom tab to Activity.
Activity merges the previous Shared 4th tab (which has been official for all since August) with your Notifications (which until now are hidden under Library -> your profile picture -> Notifications). So there are now 2 top tabs under the 4th bottom tab, yay! The move does make sense though since you can see all your active YouTube shared video discussions with friends as well as catch up on all the recent uploads, live broadcasts, and activity from the channels you follow.
Shared 4th tab before (left) vs new Activity 4th tab (right)
The interface has apparently been standard on iOS for a while - probably since YouTube sharing was introduced to the platform - but it's now coming to Android. We don't see it on our devices despite running the latest YouTube version, so it must be a server-side switch and odds are some of you have had it for a while.
You can grab the latest YouTube version from APK Mirror to try to improve your odds in getting the design, but I wouldn't hold my breath on it. You'll get it when Google deems you worthy of getting it. Meanwhile, enjoy digging through menus to get to your notifications.
- +Manish Yadav