Furthermore, if developing in Unreal 4 is so hard, why not just stick with Unreal 3?
Because the mentality is that the gaming world (at large) is of the opinion that if there exists better graphical rendering means than what you've already got, you must use it and press it as far as you can, or gamers (at large) will deem the game graphically inferior to the competition and thus gamers (at large) are more likely to deem your game a bad game on those grounds alone. I've seen it happen before, so much so, it literally does make more marketing sense to just use Unreal 4 rather than 3 just so to avoid it.