If the rights holders enter into a contract and pay you for your work, I don’t think they can turn around and sue you for making a profit off of it. I’m no lawyer, but I don’t think the law is that far gone.
As someone who couldn’t tolerate F3/NV because they paled in comparison to 1/2, I actually really enjoyed 4. It lost a lot of the engine jank and felt like a decent FPS with light RPG mechanics, wearing the aesthetics of Fallout.