Studio Display 4K resolution option missing

The options between 2560x1440 are Retina ones, where the Mac draws on an internal canvas that has twice as many pixels as the nominal "Displays" resolution, in each direction.

So

• 5120 x 2880 (no Retina scaling, lots of workspace, but also unreadably-small text and objects)
• 3200 x 1800 => a canvas with 6400 x 3600 pixels, whose contents are downscaled to 5120 x 2880
• 2880 x 1620 => a canvas with 5760 x 3240 pixels, whose contents are downscaled to 5120 x 2880
• 2560 x 1440 => a canvas with 5120 x 2880 pixels (exact Retina scaling)

Note that the Retina "3200 x 1800" setting implies an internal canvas with greater resolution than the LCD panel on an actual 32" 6K Pro Display XDR.

If there was a Retina "3840 x 2160" option, it would imply an internal canvas with the same width as an 8K display. While the relationship between the greatest Retina scaling mode and advertised resolution limits isn't always clear, your M4 Pro Mac mini supports a maximum resolution of 6K over Thunderbolt. So it might simply be the case that the internal hardware is not there to support a Retina 3840 x 2160 (internal canvas 7680 x 4320) mode.

You paid extra for a display with a 5K native resolution. Don't set it lower.

If you need better readability, use SCALED resolutions, and choose larger text:

When you select full native resolution and SCALED, graphics are drawn at full native resolution of that beautiful display.

When you select larger text, text is drawn into an off-screen temporary buffer at double size, then scaled down for display on the screen. This retains sharp edges including diagonals without stair-step artifacts.

The "looks like" resolution (sometimes indicated) is the EFFECTIVE resolution of text, after scaling.