Besides the monitor Hz not changing as the other guy said, generally the image quality is decreased. A monitor's optimal image quality is at native resolution. I speak from experience on that. I have a Dell 27" 1440p monitor used for PC gaming and connected my PS4 to it running at 1080p. The menu and game graphics looked inferior (more fuzzy) than compared to when the PS4 is connected to it's normal 32" 1080p TV that I use as a desktop monitor for it.