Have you had a look through existing threads, as the new ATV has been fairly well discussed here on GZ. I found this thread on just the next page of threads of this forum:

http://www.geekzone.co.nz/forums.asp?forumid=76&topicid=180506

Its a completely different device. On the software side it has an app store, a much nicer graphical user interface, its a lot snappier than the ATV 2 or 3. It allows you to easily switch between several iTunes accounts, so you can easily install US, UK, Australian and New Zealand apps (as well as Norwegian, in my case).

On the hardware side it has pretty good grunt (using the A8 chip you find in the iPhone 6/6 Plus), has internal storage (32 or 64 GB). It does lack an optical out for sound, which sucks.

The new remote is pretty nice, with a touch interface.

I was using the Amazon Fire TV as one of my main streaming devices (with the Apple TV 3 being used for US iTunes rentals etc), but after I got the ATV4 its been the only streaming device I am using.

I got one for Christmas, and love it! The expansibility via the App Store is awesome, and the Siri integration (which is easy to turn on, even though it's not supported in NZ) actually works quite well. I particularly like the "What did he/she say?" function, which rewinds the video by 10 seconds and temporarily turns subtitles on.

And now that the Remote app supports the new ATV, typing on it is no longer a PITA (the new keyboard is quite a backwards step from the old one). 

My only complaint is that they ditched the optical out. But this is only because of a flaw I have in my particular setup, so hopefully it won't affect you. Our Panasonic Plasma TV suffers from a bug where audio fed in via HDMI inputs, and output to our AV receiver via optical out, slowly gets out of sync with the video. This appears to be a fault with the TV which has been widely reported (the last generation of Panasonic plasmas, which are pretty much perfect in every other way). It's been fixed in the US version of the firmware, but no fix for us down here.

I also have an older AV receiver which doesn't have HDMI inputs, so I've got no choice but to go with my current config. With the old ATV, this wasn't a problem, as I was able to workaround the bug by outputting its audio via optical out.