You may already know this, but whenever someone brings up Plex I feel compelled to point out that you could use Plex on the older hardware by using the Plex app on your iPad/iPhone and then using AirPlay. When you do that, the network stream gets picked up entirely by the Apple TV, and you get great PQ and no CPU usage or wasted network hops on your iPhone.
So the only extra thing you're really getting with Plex on a 4th gen is a UI to scroll around with using your remote. For some people that may still be a big deal.
Agree. I do this all the time for our ripped DVDs and Blurays since the old iTunes "home sharing" feature has atrophied to uselessness in the past couple of years. Works great.
Still, I'd like to see that Plex-based content searchable (which I think is still not the case in the ATV4) and accessible in the "main" AppleTV interface. Just not enough to replace a perfectly working ATV3 with one. Still, when that ATV3 starts having issues (it is 3.5 years old, so might happen one of these years...) I won't shed a tear while upgrading to the ATV4.