I had to use the "battletechlauncher" as opposed to the regular launcher / opening through Steam to get it to work. I selected windowless mode and changed the resolution to 3840 x 1600. The rest of the forum posts on this topic were totally useless, so I had to figure this out myself. 4K vs 1080P really makes no difference for this game, which is a bummer. Divinity Original Sin 2, which was released around the same time, looks WAY better in 4K.