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It is both polishing and refining the edge off of the previous grit. It improves the sharpness. If you were to try and get it sharp with the 8k only it you would have a full beard by the time it was...

I prefer having both grits. The 4 to sharpen and the 8 to refine and polish. If you go to the SRP Wiki and check out the honing section's tutorial on pyramids it is an effective way for honing...