High resolution textures and detailed models coupled with good optimization that makes it run like a charm maxed out at 4k.

I don't have very good screenshots available at the moment unfortunately but this should give you an idea of what's possible:

http://images.akamai.steamusercontent.com/ugc/10059888168078...

http://images.akamai.steamusercontent.com/ugc/10059888168007...

Here's Dark Souls III which looks pretty great at 4K but unfortunately doesn't manage to run at a solid 60FPS on my GTX 1080:

http://images.akamai.steamusercontent.com/ugc/84843325410407...

And here's an older game, The Witcher:

http://images.akamai.steamusercontent.com/ugc/84843969116367...

Those hold up pretty well at 4K. In contrast I present to you Wolfenstein: The New Order which often looks very mediocre and yet manages to run worse than DOOM:

http://images.akamai.steamusercontent.com/ugc/10059888168132...

Note the low polygon count on some of the models and the very low resolution of certain textures.