almost 4 inches shouyld be fine, esp. if it is fine grained
, some experts say 3 to 4, others say 3.5 to 4, and others say that 4 to 6 is the way to go, even thos with varied opinions all agree that some is better than none, and more is better than less, but almost 4 inches as mentioned should be fine, remember that this is not a science, yet so far this hobby is all based on trial and error
however, if you do decide to add more, i would not put it under the rock, my sock is on the glass and the sand is around the rock
HTH