Few pics of boards I've been making, the first 2 are 7'9 x 29, haven't surfed it yet, I only just gripped it up. 3rd pic is 7'10 x 30, been using this as my allround board, replaced my starboard 8'5. Last pic is 7'6 x 30, super loose but bit slow, needs a fast punchy wave to get it going. All 3 are 4" thick with 2 layers of 6oz on top and 1 layer of 6 oz on bottom. weight seems to vary from 7.2 to 8.8kg (7'10 has 2 layers of 6oz on the bootom, added a bit of weight)
Yeh 4" seems plenty thick enough, I weigh 80kg and can paddle my 7'10 in pretty much any conditions, except when it gets too big. I've had mates that weigh in the 90kg+ range paddle and catch waves on the 7'6. IMO I reckon you get more stability from width not length or thickness. Having a bit of width in the nose seems to help.