6000/8000 seems awfully fine for kitchen knives, unless you're into sushi in a big way. If the knives are top notch laminated Japanese jobs, fair enough, but if they're more in the Henckels or sabatier line I'd be inclined to go for a harder stone. (The folklore is that the harder the steel, the softer the stone you need - there's a grain of truth in that one).
Japanese waterstones take a lot of maintenance. They need lapping constantly to keep them flat, though that's not too much of a chore because they're so very soft, but you need either a diamond plate or a good stock of wet 'n' dry paper. They're also easy to nick if you get the angle wrong or use too much pressure, which probably sounds familiar.
As regards grit ratings, 6000 is as far as I ever bothered with for woodworking purposes, and that used to include planing satinwood, spiteful burrs and all sorts. 8000 is well on the way to finishing a razor, and if you put that sort of edge on a kitchen knife you will never squash a tomato again, that's for damty sure. The price for that stone seems good, even including carriage, but that said, the jump from your 4000 to an 8000 is not unreasonable, so you could omit the 6000. In that case, you might find a suitable stone at a sensible price closer to home.
Alternatives for use in the kitchen might include various Arkansas or ceramic stones which need little maintenance, or a Norton - either an inexpensive India combination (not so fine) or the well regarded 4000/8000 beloved of some razor honers. For what it's worth, I do my kitchen knives on a 600 (sic) grit diamond plate and touch up on a steel. It's possible to shave hairs off one's arm with a blade sharpened on that, but it takes practice and a light touch.
Any help?