When they say “… a modified version of Unreal Engine 4 was used”: 1) … what does that mean? 2) … which has been modified 3) … how was it modified? 4) … why was it modified? 5) … can I modify it?
For example, Days Gone: “… the game uses Unreal Engine 4- a modified variant of the game …”
It means exactly what it says. They have modified/changed (in some way) an aspect of the engine to better suit their game/requirements. The changes can be minimal or significant. The source code for the engine is available on GitHub; you are free to change anything you want. There are many reasons to modify the engine, from feature-requirement to performance optimizations, or changes to the renderer to achieve a stylistic impossible otherwise. What precisely the developers of Days Gone have done I can’t answer, but it’s very common (modifying the engine). Having the source code available gives you this freedom and it’s awesome.