You dont want to mix 100% of it all of the time, no. Buuuut....
This is a hotly debated topic, and a lot of people have opinions, some I think harken back to the "dark ages" of reefing, where we were equipment limited not necessarily knowledge limited.
In my opinion 4" isnt all that deep but if you look at the people who are having success, such as Paul B's "reverse undergravel" or Lasse's sandbed plenum, they're having success because the sandbed does not become stagnant.
