Light riders can use a bigger sail on a given board, while a heavier riders weight will overload the board (together with the rig) and make it behave like a pig. In the same wind a heavier rifer of course needs a bigger sail, I typically use 9.0 while smaller friends use 6.5 - 7.5. But they would be able to take a 9.5 - 10.0 on my board in very light winds and make it work.
My smallest board is a RRD FSW 94, and at my weight of slightly more than 100 kg it is quite manageable in overpowered 4.2 weather (gusts of 50 knots +). I am sure that you will enjoy a K86 with a 4.0. Don't sit them out!