I'm using the 4K output of my Xbox One S. So either Blu-ray is upscaled to 4K before it goes to the Epson, or it's native 4K from a UHD disc.
Feeding the Epson 1080p doesn't yield the best results, and now that 4K players are such high quality and well priced there is no need to if you are investing in an expensive projector.