In addition to the new watchOS and tvOS betas, Apple also released the fourth beta of OS X 10.11.4 El Capitan. Again, this particular beta has no new user-facing features, apart from the ones from the previous betas.
There are no known new features in the fourth beta.
From the previous betas:
- Live Photo viewing and sharing in Messages (via iMessage of course, not SMS/MMS) (carried over from beta 2)
- Notes.app — the ability to secure individual notes with a password (carried over from beta 2)
- Notes.app — the option to import notes from apps like Evernote and similar, or from rich/plain text files