Let's hope they did not code themselves into a corner and the X4 engine has many more years of life in it.
X3 ended up as good as it was because it was an evolution of X2, which itself was an evolution of the original engine. Because it had several releases building on top of each other in Reunion, Terran Conflict and Albion Prelude. Because its long lifecycle allowed people to learn how the engine works and have the time to write frankly astonishing mods for the game.
This is what X4 needs to become to truly succeed it.