Since my Dell only has one HDMI display output, I've replaced the previous 1080p Paul monitor monster hand me down.
I didn't think my laptop would let me do a built in 1080p and 4k monitor because it's so old and limited to HDMI 1.3. But thanks to the magic of Linux configuration, I now have the builtin 1080p at 60Hz and the 4k displaying scaled 2k at 30hz.
I couldn't figure how to add custom resolutions with Wayland and Mutter in Fedora 25 so I've reverted back to Xorg for now.
Here's how I did it with xrandr and cvt:
Run cvt verbose with your desired x and y resolution and refresh rate. It retuns a list of X settings along with the calculated bandwidth it needs to display. This helps you figure out if your output can carry the signal. in this case I was conservative with 2k at 30Hz.
cvt -v 2048 1556 30
Take that output and pipe it to xrandr --newmode. After that you add that mode as a x setting for an output device with --addmode. You can then set the mode with --output or in the Gnome display settings.
xrandr --newmode "2560x1440_30.00" 146.25 2560 2680 2944 3328 1440 1443 1448 1468 -hsync +vsync xrandr --addmode HDMI1 2560x1440_30.00 xrandr --output HDMI1 --mode 2048x1440_30.00