I want to resize windows into partitions that are four vertical strips equally dividing the monitor. For example the 2nd and 3rd windows in the screenshot below.

Is there any way in DisplayFusion to set up custom monitor splits?

Keith - thank you. That helped me solved another problem but is not helpful to what I'm trying to do here.

I want one application's window to fit to the right 50% half of the monitor. I want another application's windows to fit to the 3rd and 4th 25% vertical quarters of the monitor. Since these overlap, I can't use monitor splitting. It's really window-resizing that I want.

You have some great window resizing features in Settings - Functions. (e.g.: Move Window to Bottom-Left Corner and Size 50%; Size and Move Window to Left Side of Monitor) It's these I want custom version of.