AFAIK PS4 Pro only supports 2160p or 1080p, nothing in between. So you would get 1080p output.

VegetaCreeper
Not true Mate...

Yes true mate. You are confusing the internal render resolution (1440p for Uncharted 4) with the output resolution from the PS4 Pro. Uncharted 4 renders at 1440p and that is upscaled to 4k on output (much like how many ps4 games are rendered at 720p then output at 1080p).
 
You can see the output modes of the PS4 Pro here: https://www.playstation.com/en-gb/get-help/help-library/system---hardware/sound---screen/ps4-pro--how-to-enable-4k-resolution/
720p, 1080i, 1080p, 2160p (yuv or rgb) are all that is supported. If you hook it to a 1440p display and set the PS4 Pro to 2160p you will not see anything on the monitor, have to go down to 1080p.
 
Here are other people confirming: https://www.reddit.com/r/PS4Pro/comments/5sdkb3/1440p_monitor_with_ps4_pro/ "Person with 1440p monitor here! Games will only run at 4K or 1080p, but games running at 1080p with higher internal resolution can still look better than on a 1080p display. At least that's what I feel like when playing."
 
https://www.reddit.com/r/PS4/comments/5cz8eh/anyone_using_ps4_pro_on_a_1440p_monitor_how_is_it/ "From what I've read here, it'll only output to 1080p. You'd still get the improved 1080p downscaled picture but no resolution beyond that." "Yup understand it will only do 1080 or actual 4K"
 
https://www.reddit.com/r/PS4Pro/comments/5or6ms/has_anyone_tried_using_ps4_pro_on_a_1440p_169/ "Exactly. It will only display 1080p unless you have a 4k tv (or monitor)"
 
I can go on if needed but I think you get the point.