Black bars on side of screen

[Delta 6]

I'm playing on a 4K screen but am using a GTX 970 so I don't have enough horsepower for 4K gameplay. So I set the resolution to 1920 x 1080 and now I get a smooth 60 fps with vsync. However, I now have fairly large black bars on the left and right sides of my screen. These are present at all resolutions except for 4096 x 2160 (but it runs below 30 fps at that setting). I can tell this is an in-game issue because Steam's in-game FPS counter is still showing on the bottom left of my screen, inside the black area. I tried adjusting the FOV but all that does is adjust the FOV within the rendered area. It doesn't affect the black bars.
I've played dozens of games on my PC and have never had this issue with any of them. Does anyone have an idea how to fix this?