So many questions here and I don't know how it is even possible to suppose that Epic and Steam has different game builds...
I must say that I have exactly the same issue as yours.Oh, and here are screenshots of my recording software with both instances. It has a setting for expected input resolution and is set to what the game resolution should be - so with the Steam instance it takes up the full window in the preview panel. With the Epic instance it expects "full size" and displays it smaller. The visual comparison in the original post is one thing but to me this seems to confirm that the Epic instance is actually a lower resolution.
I must say that I have exactly the same issue as yours.
Did you get to resolve that?
Thanks a lot
Edit: Well im an idiot sandwich lol. All the graphic settings were the same but for some reason one was different. The freshly installed steam version had borderless window as setting, even tho it should have been fullscreen like with the Frontier version, no clue why that settings was changed as the only one. After changing to fullscreen i got my 4k picture back and it looks as it should.
That answer is not by the guy who opened the thread, and talks about something different. But as a matter of fact I seem to have resolved by tweaking back and forth the fullscreen / borderless setting.