1080p30 Linear vs 4K60 Open
here's the thing, and it seems people don't get this:
If you'd want to play any 1080p30 game that pushes the PS4 to it's limits like TLOU2 at 4K60, you would need 4x the GPU power alone to go to 4K, and you would need 2x the GPU power to go to 60fps.
this mean, simply running a cutting edge PS4 1080p30 game at 4K60 you would need a system that is 8x as powerful, that is a 14,72TF GCN GPU. And this is without a single improvement to the make-up of the graphics (assets, effects, draw distance etc.)
as an example, this is an open game that often pushes the base PS4 to its limits at 1080p30fps, it has a lot of dips that are GPU related.
in order to get this exact game, same as it looks right here, no additional improvments whatsoever, running at 4K60, you would need said 14,72TF GCN GPU
in this game, shadow cascades change resolution literally 2m ahead of you, it has really harsh shimmering due to a low quality AA solution... and it would still need most likely the full power of the PS5 or even the Series X to push it to 4K60