What material to use to fill 4' x 4' window hole for soundproofing?
I recommend going to the home centers and purchasing a  4'x8'x2" sheet of expanded polystyrene.  This is a very good sound deadener. Cut it to fit tightly. It costs about $20.  It is also a great heat and cooling insulator.  It will totally block the light.  It is extremely light weight.  If you take care of it, it will last forever.  Easy to take in and out.  Easy to store.  Plug any cracks with rags and tissue paper.