DSB in a short tank

[2poor2reef]

In my experience if you are using oolitic sand then 4 inches is fine for a dsb.

 

[GLD]

In my excitement to install a deep sand bed, I forgot that I have a short tank. My 55-58 gallon tank is 18" wide, 36" long & 21" tall.

I already have purchased enough sand to build a sand bed 6" deep, but am wondering if I can cheat a little. Is 6" required to achieve the benefits of a DSB, or can I rely on 5" ... or 4"??

 

[GLD]

4" certainly is the answer that I am hoping for, but I don't want to bias the jury. Any more depth suggestions before I start pouring sand?