Hi, Bob. Thanks for your question; I would not add 4 to 6 inches of soil. I think you can get away with a couple, but woody plants, in particular, don’t like to be planted too deep or smothered later in life this way. Have you seen the article I did on the blog about underplanting? I try to to tuck things in here and there and add maybe an inch or two of soil and an inch or two of fine-textured organic mulch (let’s say a combination of 3 inches total). The mulch breaks down in time so isn’t as problematic as soil. The underplanting article: http://awaytogarden.com/10-thoughts-on- … erplanting See you on the forums and blog! M.