That is interesting. You can't emulate a 6K that way, though.

When you use the 4K you leave a 4K stration pattern. The 8K only cuts half as deep, so it will striate only the high areas on the edge until you've done enough passes to remove the 4K troughs. That would take a lot of 8K strokes.

On the other hand, one or two 8K strokes may not be enough to put down an 8K pattern on the high spots. Why you notice such a difference going from 2 to 3 to 4 I don't know, but it seems that you're putting down a pattern of 4K troughs with an 8K scratch pattern on the high spots between 4K troughs.

Too few 8K strokes don't put down the pattern and too many take down the high spots too much.

Traditional thinking would be that you should get the best edge when it's all 8K striations.