Getting best 4K experience on my new Sony TV

Att offers no regular 4K programming. Any 4K  content would be through streaming apps or Apple TV type devices. A 4K capable hdmi cable would be needed from an Apple TV, 4K disc player, etc. Settings in the tv may need to be set to 4K. Your tv owners manual will cover that. 

U-verse will show the stream quality that you have selected, either SD or HD.  There will be no fall back due to signal issues.  So as long as the Wi-Fi is capable of carrying the stream it is no worse than wired.  The 6 Mbps max stream bandwidth of U-verse TV HD signal is not a real challenge for Wi-Fi unless you have interference, long distance, etc.

U-verse equipment has a maximum output of 1080i.  Your TV would upscale that as best it can.

Congratulations on buying a nice TV, but you’re on a Uverse forum and there is no, and never will be, 4K anything on this service.   

As stated any 4K experience from non 4K DVD player in your home will be from the internet not UVERSE IPTV.

Therefore the question becomes what internet speed are you subscribed to? 1080 HD streaming uses about 5.5M to 6Mwith recommended is 8M per stream.... while 4K uses about 3 times as much with recommended of 25M per stream.

If you have internet 25 as best available, steaming a single 4k content  means not much bandwidth exist for other devices in use at the same time, including UVERSE IPTV receivers watching or recording programming as IPTV gets preference in bandwidth usage with balance being available for internet. 

Example... 32M profile with internet 25... each IPTV HD channel in use consumes about 5.7M that will call 6M... if (2) HD channels in use (32-6-6) leaves 20M max available for all internet devices during this time frame.