Obtaining 2560 x 1600 on a 3840 x 2160 with surrounding bands
You cannot request this from the computer, as it would have to support the full resolution in order to do that. This is a feature that would have to be provided by the monitor.

You’ll have to check the manual for your monitor to see if it can do this. Most monitors cannot.

On the other hand you’re not stuck with that one specific setting. You can choose to use the full 2560x1600 resolution, but it will typically fill up the entire display.