Most reasonable way to raise desk by 4 inches
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.