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I probably would rip a 2x6 down to 4 inches, make a three sided frame out of that and put the desk on top of it. Then attach the desk to the frame with some pieces of metal called "mending plates". make the inside dimensions of the frame the inside dimensions of the desk. Paint the new wood and plates black. If you wouldn't like the look of 2x4s at the bottom, you could cut some strips out of 3/4 plywood (or whatever thickness the desk is) and attach those to the desk bottom in some way, like more mending plates or pieces of wood.
You could Isherwood's idea, only use 2x4s instead of aluminum tube. Probably be a cheaper.
There are also "bed risers". They re small cone line platforms. It would take 4. I personally wouldn't like this because it wouldn't be that stable.
You could make or buy a set of risers. These are commonly sold for desks and beds, and are available in a variety of styles/materials.
Here's one set
Alternatively, if the feet on your desk are a standard screw, you could just buy a set of legs (like for a couch) and replace the bottom pads.
I wanted to improve my desk setup and make it look clean like those videos you see around about productivity and stuff like that.
But man, desk organization accessories can get really expensive, and I feel like unless you are looking for the super premium hand polished by the elves craftsmanship quality, the price can just be ridiculous.
So I decided to make one myself
The shelf is called BERGSHULT. It is 80 cm x 20 cm and 20 mm thick, and costs €13 here in Italy.
I simply designed the legs using #fusion360 to match the height of the books I had been using. Then I printed them laying on a side, using 4 perimeters and 20% grid infill to make them sturdy (probably overkill).
I fixed them using self-tapping screws, after drilling pilot 2.5 mm pilot holes using a piece of tape on the drill bit as a guide to avoid going too deep.
I can share the part files if anyone is interested.