I personally think the ATV3 is better in many ways. If you had one already, I would say stick with that for a while. But the ATV4 is the future so for a first time buy, get that.
I was really excited about the ATV4, but after using it, I find myself getting a little frustrated with the bugs, the new video apps UI, and the remote.
I think a lot of the stuff will be fixed eventually, but until then, I prefer to use my ATV3. If I could get the games on the ATV3 UI I would love it.
For some people on the forum, mentioning the ATV4 flaws is a really touchy subject for some reason, but I will name a few.
The bugs, including random restarts, and some audio problems.
The remote, I think it was poorly designed, meaning button placement. Also it is easy to accidentally press the wrong button, or swipe the touch pad and RW or FW a movie you are watching.
You can use a IR remote with the remote learning feature on both the ATV3 and ATV4. But the buttons are inconsistent on the ATV4. Example, the select buttons doesn't play/pause on all video apps. And sometimes the pause and play buttons, if you program the advance IR remote buttons, they do not always work on certain apps.
The video apps, especially Netflix sucks. It visually looks nicer, but the organization is strange. How can you pick a genre now?
Hulu on the ATV4 is actually set up a little better than the ATV3 version, so thats good.
People like to point out the faster hardware on the ATV4 when compared to the ATV3, but I haven't noticed the ATV3 ever being slow. Hulu on the ATV4, while having a better interface, the initial start up takes 3 times as long.