The sine ratio involves the opposite over hypotenuse.
With respect to reference angle J, the opposite leg is the one furthest from the angle. So the opposite side is KL = 4. The hypotenuse is always the longest side, always opposite the 90 degree angle, so LJ = 8 is the hypotenuse.
I'm a little confused by the wording of the question. If 3 pieces of the pie are shaded the fraction would be 3/8. I hope that's what you meant have a great day!