Here we go again with these lfs people..
First off, hes right on how live sand NOT from a bag will seed your rock, AND your sand. But about the depth... NO. You dont NEED 4" for a reef lol. I've seen some bare bottom reef tanks, so i would think that you can have however much sand you want. It is all going to depend on whether you have livestock that like to burrow IN the sand, or not.. Then you will need a decent amount of sand for their needs. 2" or more, depending on what animals ofcourse. Usually gobies like 3" or more, but i hear that they will pile sand up to suit their needs.