And Unreal Engine 4 unless you have something stronger than a Titan X your computer will EXPLODE.
Actually, the reason Unreal 4 games require higher performance is because the people who use Unreal have the budget, people and time to make more detailed graphics, and unlike Unity, Unreal has the capability to actually handle the high level of polys AAA devs tend to throw at it.
Unity games are generally developed by people with smaller budgets, people who don't really care for adding polish to their stuffty simulator games, and people who find that Unity can't exactly handle high-levels of detail since it's not optimised for those kinds of operations.
EDIT: Okay, so maybe the specs are a little higher, but that doesn't null and void my point. You can very well get a game built in Unreal that runs much smoother than it does in Unity. It really depends on the content, and how you've programmed your games. Throwing a lot of redundant or slow-loading operations in will of course make your game run like stuff regardless of engine.