Dan - IMO if you're happy with your 1K, stick with it. From what I've read here and seen on Ken's videos, the Nubatama Bamboo 150: http://www.chefknivestogo.com/nuba150grwa.html
would be a great coarse stone. The Suehiro Rika 5K is one of Melampus' favorites and is very nice to use. The finish is said to be around 3K, so that might fit very well in your progression. As a last stone before other things like leather strops, it leaves a sharp, toothy edge. You could refine that edge just a bit more on the 6K if you wish, for a next step before stropping on other media, if you do that at all.
Another option might be the new Nubatama Ume 2K stone as an intermediate step, or a finishing step for soft/Euro steel. IMO a 1K, 2K, 4K, 6K progression is overkill unless you're trying for that super nice, mirror polished look.