RE: Kombat 86 with very small sails ....
I weigh ~ 80Kg
I use a 4.2 most of the time with my K86 (+ freeride fin) and it rocks! Don't think there will be a problem with a 4.0 (range is stated as 4.0 - 6.0). I ordered a somewhat smaller fin (actually the fin belonging to the new K87) because I still plan to use the K86 with a 3.7 and I'm quite sure (speculation) that it will handle well.