Don't use 3 or 4 inches of sand.
Either use only 1" for looks and an AEROBIC bacteria bed
OR:
Use AT LEAST 6" for a Deep Sand Bed (DSB)
3" is just enough to provide a nice AEROBIC bacteria bed and also just enough to trap waste and other crap in the tank. But it's not enough to do any real break down of nitrates. You need 6" to get any ANAEROBIC bacteria and subsequent nitrate break down.
Lots of people don't want to give up 6" of space n the bottom of their tank. Thats' understandable. In that case, just use 1" of sand. 2" or 3" or 4" isn't any better than 1". It's actually worse.
My tank is 24" deep, so 6" of sand in the bottom is fine with me. I kinda need it because I only have 144w of T5 lighting and I've been told it takes a LOT of lights to penetrate way down that far. With 6" of sand in the bottom my lights only have to penetrate 18" to reach the bottom.
Option 1: 1"
Option 2: 6"
Setting yourself up for failure from the very beginning: 2", 3" or 4"