Having just done this i've got to say it's a mixed bag. I had a 55" Sony 1080p and upgraded to a 55" Sony 4k - x85 with Android tv. I already had a sony amp - STR-DN850. The upscaling of the xbox is superb, especially with the Vivid default picture setting. Normal tv and movies i change it to the standard picture setting after fiddling with some of the advanced settings.
Blu Ray's through the xbox are excellent, the picture is stunning!
The Android tv isn't great, it's ok.. Youview (in the uk) is slow and cumbersome and sometimes craps out although the picture quality on freeview is excellent.
Netflix and amazon prime UHD quality is very good, not sure it's financially worth it as the 1080p picture quality was also very good. UHD brings another dimension to the picture but only in long shots and panoramic vista shots. Close up conversation and action sequences are no different to the 1080p picture. Also, be aware that you are restricted to using the default applications on the tv for this. When watching Bosche on Amazon Prime UHD for example it's only when the picture changes to a long shot of the city do you notice that you are watching 4k. These shots in this show and others are far and few between. Other than the Amazon Fire Tv 4k and maybe one or two others, nothing else will allow you to access 4k content.
Text based content, youtube, the menu screens on the tv etc are all fantastic.
If you're going 4k you need to look into HDCP. 4k content requires this and if you have a new Amazon Fire tv 4k for example, this has to plug into the tv as the HDCP does not work with the amp. The new Sony amp DN860 only has one input that supports HDCP. (I know this is supposed to be about the tv but this is my experience) i've learned a lot about HDCP and 4k in the last couple of weeks...!
Where the 4k does fail is watching standard definition content, either normal tv or streamed content, lots of jagged edges and washed out colours. DVD's are also noticeably poorer even with the upscaling. All are watchable but you do know that you're not watching HD.
I have a lot of ripped DVD's that i access through Plex (there's a Plex client downloadable from the Google store). Picture quality on these are noticeably poorer than on the 1080p screen.
But, there's something great about having the latest stuff. The tv with Blu Rays and UHD content is a head turner. the quality is superb, but then so is blu ray and HD through a decent 1080p tv.