I just purchased an external display monitor for my iMac (27", Late 2013, old I know lol). The external display is a Samsung 28" UHD Monitor (U28R55). I'm having issues matching the display resolution.

When I restart the monitor, it says "Current Resolution: 1920 x 1080 60Hz, Optimal Resolution: 3840 x 2160 60Hz"

However, when I go to System Preferences > Display, and choose Default for display, the resolution is super tiny compared my my iMac. My iMac shows 2560 x 1440, however the external screen can only go up to 1600 x 900. I'm confused as to why and if there's a way to fix this?

I've already returned one other monitor, should I return this one as well? I'd like a resolution that matches or is at least close to the iMac.

Any advice appreciated!