My leader lengths depend on the weight and type of lure fishing I'm doing.
I am assuming that we are talking about lure fishing here. My heavier gear has leaders around 1-1.5m.
My lighter rods have more and my big plastic rod has only 600 - 800mm.
I always use a flouro-carbon leader but I am sure that sometimes this is over kill.
I fish mostly for flathead and feel that if I get below around 1m I start getting less strikes, particularly in water less than 1.5.
Water colour/clarity though plays a big part.
Can't help but thinking though that at 4-6 kilo your aiming for Jews and bigger flatties in which case, I have been hammered lately on 15 and 20lb leaders and I am now fishing 30 on my 4-6kg rod.
Can't tell you how frustrating it is laying in bed thinking what if?
As hazza suggested find a good knot to match your braid and leader as the braid will cut through really quick.