I still don't even know what the 4th gen does that my 3rd gen can't. Games? Ha.
As far as apps are concerned, I don't think that there is anything the 4th gen does that an iPhone w/AirPlay + a 3rd gen can't do.
Not an argument, just a data point: beyond the standard Netflix/Hulu/YouTube, I commonly use Reuters TV, The Bat Player, TWiT.tv, WWDC, and Infuse. Plenty of other apps are there, of course, but those get the most use. And having things like the NASA app there is cool (hmm, just turned it on, they've got a countdown going for the launch of a space probe headed to an asteroid).
I upgraded from a 2nd gen and YouTube (or maybe it was Netflix?) on that had given out and wasn't slated to be fixed. Yes, I
could AirPlay much of that from my iPhone, but having it all on-screen is simpler and more convenient. And since I "cut the cord", in favor of Netflix and Hulu (saving a ton of money), the AppleTV is the only interface I use on my TV (which could just as well be a monitor now).
I'm also looking forward to next week's release of the new version of tvOS with HomeKit support - I've got some Hue lights in my living room, and being able to tell Siri on the TV to adjust the lights, with the remote that's already in my hand, will be nice.