The 8K is very similar to the Kitayama. It's soft and finishes well but I tend to strop only on it. It doesn't need much time to soak at all but this is somewhat common on high grit stones.
The 4K leaves a good kitchen knife edge. It's a pretty good cutter and doesn't load very fast. This one needs some soak time.
If I had to choose 1 I would get the 4K. http://www.chefknivestogo.com/im4kst.html