So from what I see Beta 4 was a complete disaster. Siri barely functioned and the watch would disconnect reconnect from phone. Also it would randomly not turn on. Some new stuff I found (also could be from previous betas) New Butterflies, Flowers and Jellyfish, the 70 seconds (Odd choice)
Things not working: Timelapse Freezes constantly, Hey Siri doesn't work, Some Apps crash (normal). More stable still some bugs.
I cannot speak for apps crashing - as I have not used many this morning, but I can say that following the upgrade and a full reset (set-up as new watch) - "Hey Siri" works, location works, heath data syncing work, battery is not noticeably faster to drain (vs. 1.01.). This was my first plunge into the WatchOS 2 waters and after the reset I am quite pleased. (I might even say that wrist detection is faster than I've seen it before.)