most of the time, dvd players are smaller in footprint than receivers (and the players look too flimsy to be able to hold lbs upon lbs of receiver or amp)
in my case (player over amp), I put in aluminum tubular feet (four corners) to encourage air circulation front to back and left to right. tubular feet so even the heat conduction between the amp/receiver below through the feet can easily be dissipated too.
4" space is indeed ideal, but maybe you can get away with a smaller space if you put in silent PC fans.