I think a lot of it depends on how big the aquarium is. For instance, 4 inches would look fine in a 90g but ridiculous in your 8g biocube.
Plus, there is always the issue of anaerobic pockets developing in too-deep substrate. In a 8g, I think 1.5" should allow for sufficient rooting without compromising appearance or volume of the tank or encouraging anaerobic pockets.
from personal experience, i've found that the min depth of the substrate to about about 1.5" in the front and close to 2" in the back (if not more). anything less and the plants seem to float a lot.
I would start with 1.5" and see how well you can plant the tank, if you find that it's too hard to plant in as you keep hitting the bottom glass, then just add more substrate. It's easy to add later on. I tried running 5" of substrate in my 10 gal once, it looked really funny.