I will try my best...

looking at this largely-complete online version of the book (I like to use this to keep my pristine hard copy of the book pristine :)

http://books.google.ca/books?id=wCKVrAc94TcC&pg=PA45&source=gbs_toc_r&cad=4#v=onepage&q&f=false

The section I'm working on starts on page 57, "Positioning the Deck Beams"

The 4" measurement mentioned causing me the confusion (that I think I will ignore as it seems to make no sense) is at the top of page 59, right column.

Since I am not adding 4" for a seat (pg 57), I don't think I need the extra 4" (pg 59)

Let me know if that doesn't help.

Thanks

Jamie