Unreal Engine 4 is similar to Unity, but concentrates more on graphics, optimization, lighting, etc.
It is fairly new (about two years old), and as of now there has only been one actual game created, called Ark: Survival Evolved. However, it has a ton of potential, and unlike other games with really nice graphics, this engine is optimized enough so that there is little or no lag! One of the biggest features over Unity, however, is the landscape designer. This is a feature sorely lacking in Unity. You can easily design landscapes and "paint" them for different areas, like mountains, paths, fields, etc. Not only that, but you can also "paint" on foliage! You can choose what kind of foliage will go into a paintbrush (models/materials are produced by the developer) and just paint where you want it to go. This makes grass, trees, bushes, etc a lot easier to place, without having to worry about placing hundreds of models.
Beside the engine itself, however, Unreal Engine heavily supports game development and student learning. It is completely free (no "Pro" version, though you can buy assets in the store made by other people), and you don't even have to pay Unreal Engine to use your game commercially! You just have to give them 5% royalty of your game, and you get the 95% rest.
And if you have a game you are working on, and you have a first working beta version ready, you can get a grant (Unreal has 5,000,000 USD to give away in grants) that will help you with developing your game, and eventually releasing it the public. Most would assume that there is some catch; why would someone give you money for free? To quote them:
Unreal is focused on helping developers make better games, and not on making money.
Not enough for you to download it right now? Check out some of these videos of the engine in use: