OK, I have my 4D out and I am noticing something interesting. The black marks on the tow points (red and blue) are where the inhaul stopper knots are positioned on my kite. I think maybe your bridle was incorrectly tied. The knot on my kite is 1 5/8" from the prusik knot and is right on the black marks.
I am just relaying what the Prism Tech support person told me, after he examined a 4D. Since that is a larks head knot, how do you manage to keep it from slipping off the black mark?