Solution: clearly you just need 2 more monitors ;)
Seriously though, that's interesting... not the same behavior as my dual-monitor win10 machine - might be a video-card specific treatment? When you play other games full-screen do the games see your computer as having two individual 4k screens? or one massive 8192x2160 display? (drool, ha!)
Extra strange because the game generally has a viewport area that's 16:9 - so I'm surprised that even though it's spread across two monitors... that you didn't get black bars on the left right sides.
Anyways, I'm definitely going to be adding a proper Options menu at some point with some resolution settings. Hopefully that'll address these kind of issues.
Thanks!