4.0-6.0 is a bit optimistic, but as a flat water board where high wind handling is less critical and you can use power to pin the board down it's accessible for someone at say 75 kilos. But in waves, when wave riding the sweet spot of a board get a lot smaller. So, while I (at 72kg) wouldn't mind the Kode86 with a 4.0 on my local lake, I wouldn't want to use it on a wave with much smaller than 4.7 (if I had a smaller board as an option, with good waves and 4,0 winds lining up and nothing but the K86 to sail on, I would of course quit wining and just rip it up on the K86 as good as I could).
So, these recommended ail ranges are very "averaged". If you have questions about a particular board, set of conditions, sail sizes and so on it's always better to ask and get specific advice. That's why we're here.