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If you are on a tight budget and you are doing well with the 4K you have: why look further? I don't know the hone from Amazon on the link you showed. You better not try the Amazon stone unless it...

You may find the jump from 1K to 8K a bit to high. Norton is a 4K/8K combo.

Carborundum is, I believe, a substance used for synthetic sharpening stones. Grit depends on size of the carborundum...