Here's the two images -- the first is the 4K render saved as a 16 bit tif no compression and the second a HD render also saved as 16 bit tif no compress and then re-sized to 4K. A.I Gigapixel has two filters -- noise & blur moderate or strong, but these were not used. The re-size file hase the "topax" tag in the file name. Finally, I cropped both in the latest Affinity Photo and exported to jpg highest quality (to save space on the website.)
My old eyes don't see much difference -- I see the biggest difference in the yellow flowers. Keep in mind that scenes with up close or hard surfaces may show more differences (it's something I need to test.) I have a strong GPU and it does the re-size pretty fast -- about 5 seconds. You can use your CPU if you want.
The TG render of the HD scene (the one that got re-sized to 4K) took 45m 37s. The 4K TG render used for comparison to 3h 31m 51s to that's a savings of 2h 46m 14s. If I did the math right, you could render that HD scene 32 times in 24 hours, but only 6.9 times at 4K.