I have a PS4 Pro and I switch between playing ESO on both a 4K TV and a 1080p monitor. On the 4K display, both the 4K resolution and enhanced 1080p visuals modes work perfectly. Games optimized for PS4 Pro and 4K will be downsampled to 1080p on a 1080p display resulting in a dramatic increase in image quality, even on a 1080p display.
However, ESO appears to detect wether a 4K screen is present on startup and completely disables the enhanced PS4 Pro modes if it doesn't detect one. The result is that playing on a PS4 Pro through a 1080p display is exactly like playing on a standard PS4 model. 4K downsampling is not available and neither in the enhanced visuals 1080p mode.
If I start ESO on my 4K display and then switch the output to the 1080p monitor while it's still running, the PS4 Pro automatically downsamples the 4K image resulting in a beautiful 1080p picture with excellent image quality, so clearly the automatic downsampling is working excellently. The only way I can achieve this is if I start the game first on the 4K TV and then switch to the 1080p monitor, essentially "fooling" ESO into not running its 4K screen detection pre-check.
ESO should not be disabling the PS4 Pro enhanced modes when a 1080p display is present. Both the 4K resolution and enhanced visuals modes are incredibly useful on 1080p displays.