People saying "I see no reason this could not run on the PS4".
You need to see a detailed breakdown between the 2 platform versions to really appreciate the leap between them.
Watch this Digital Foundry video on the differences between the PS5 and PS4 version of Horizon Forbidden West, it goes far beyond just "resolution and framerate".
View: https://www.youtube.com/watch?v=qVdxbaS1BiY
Everything for instance is rendering in real-time on the PS5, meanwhile the PS4 version uses pre-recorded video clips during several moments in the game because the PS4 literally can't handle rendering that stuff in the same way.
After watching this video, I believe Guerilla aren't bullshitting when they are saying that they are going to push things further in the DLC using the PS5's tech.