As paradoxical as it may seem, it is possible for a blade to be less effective at cutting arm hair as it progresses from the 4K stone to the 8K stone. Your bevel/edge may have been deficient at the 4K level, but because the 4K stone is somewhat rougher than the 8K, the roughness of the resulting edge coming off the 4K stone may have been cutting the arm hairs, not the actual sharpness of the edge. I've seen a 1K edge cut hanging hairs and not be able to do the same at 4K. I would first establish (through observation with a 5X loupe and bright light) whether you do in fact have a proper bevel and proceed from there. A 4K edge under magnification exhibits mini-serrations that the 8K stone eliminates, making it seem less sharp when it actually is sharper although smoother.