You can right click the Windows desktop, select Display Settings, and use the slider to change the scaling. Try 150%. However, JRiver still doesn't look right with some menu items remaining very small. No amount of fiddling I do with View > Size or Display Settings scaling seems to help.
With my JVC RS500 (X7000) projector, I leave my HTPC in 1080p. Then I go to Options > Video > Display Settings and set Display Settings automatic change mode to Custom. For 23.96 fps I set it to 3840x2160 - 32-bit @ 23 Hz. This keeps JRiver at 1080p mode for browsing, etc., and then switches to 4K just for the movie in Display View. Yes, you have to wait forever for the JVC projector to sync, but its okay when just watching a single movie.