Slurries tend to change the shape of the bevel ever so little. If he uses a slurry on the 4k then it would require more time on the finer grits "IF!" they are not used with a slurry.
I also suspect that this hone mfg. would require a slurry stone.
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They certainly supply a nagura with their 10k super stone, but i don't think it's that common at lower grits. Also the naguras naniwa supply aren't rated that highly by people who have them. If there's not one with it then i would infer naniwa don't think you need one, which isn't to say you couldn't try it with one.
Although they supply one with the 10k, i think the knife guys at knifeforums prefer to use it without it anyway.
Jacques, the Naniwa #10000 stone makes a great polishing and finishing stone. It's a slower cutting hone that sharpens at a higher grit. I wouldn't recommend the #8000 stone to finish though. It's not sharp enough to be a finishing hone and there are plenty of better ones available anyway. Nonetheless, it depends on what you're going to use afterwards. If you're going to use green oxide paste or a comparable paste, then I don't think it's a problem. If you're going straight to the linen/leather after the #8000, then you'd have to make extra passes on your strop to compensate for the big jump. I hope that helps. Regards.
An 8000 grit is more than fine enough to finish with, I used just that for quite some time. If I recall correctly the guys in Germany had access to only a 6000 grit for awhile and they did just fine.
Just my two cents,:)