GameInformer has a great look at some pretty cool Diablo 4 concept art up on their page. It’s worth a look!
The art showcases the darker tone of Diablo 4 which extends to the colors and materials used on various fabrics, costumes, and weapons. The horses in particular look pretty damn cool. Even the concept art for the overworld showcases this much darker tone compared to Diablo 3.
Art Director John Mueller discussed this tonal shift,
“I was really excited about the opportunity to do a really medieval-feeling world and setting. We started with going back to the source of what makes medieval fantasy cool. When you think about demons and angels and mankind in the middle of this conflict, it’s rooted in our own history, so we went back and looked at a lot of medieval art and armor and weapons … We wanted to craft like a medieval masterpiece. We wanted it to feel like something that came from this time. It’s kind of like European folklore.”