4k image downsampled to 1080p counts as "improvement in 1080p"
The easiest and quickest way for dev to make pro mode is to simply use the checkered rendering to output a 4k image. That 4k image would look better than base ps4 no matter what tv you use.
Sure... if downsampling is implemented in games (which I don't believe would come automatically), it would probably look a bit better on 1080 TVs, but that's hardly worth the console to me. There's a reason Sony isn't advertising that as a 1080 TV advantage. Like I said, wouldn't it be better if Sony mandated a 1080 mode for all games? Without that, the message absolutely isn't as clear as it could/should be.