How can I make use of a 4 x 2ft window that has drywall behind it?

If there is room for rope lights you could use “Gallery Glass” to make the window look like stained glass backlit with the rope lights.

😊

You did not mention where in the house the window is. If it is in a kitchen, add shelves and wooden dowels to make a spice rack.

Place a light behind it, cover the glass with an opaque film...

There are some beautiful contact papers that could fill in that space, maybe wall paper. If you are creative, paint the area and draw or paint a scene-similar to a mini mural.

Use for spices or special stuff.

I would add shelves, and display a few decorative items, put family photos in the panes, or enlarge a photo of a favorite vacation spot, to fill the whole space.

can you send a pic?