if your tank is only 5" deep then its a simple equation of how many inches3 u need. height doesnt matter, all i need to know is the 4" u want. heres the sum
47 * 5 * 4 = 940"3 i think (i did it with out a calculator in cm it would of been easyer but u realise 4" in height is about 10cm... thats quite alot ide say u only need 2" max which makes it 470"3 of gravel if my math is corect.