is a chart containing comparisons for various grit rating systems.
So your 1k EP stone is about a 2k Japanese stone. So if you wanted to add one stone a 4k stone would fit nicely between the stones you have before the 8k. Or a 5k would be OK too.
If you are looking for a stone in this range, my favorites depending on the steels you sharpen would include the Nubatama Bamboo 4k or 5k, The Shapton Glass 4k, Shapton 5k pro and the S Rika 5k, listed in my personal order of preference. I'm excluding the Chocera 5k Superstone 5k and Nubatama Ume 4k since you don't care for the Chocera 5k and these are of similar composition. The Nubatama 4k and 5k will give you the best feedback, the 5k pro the least feedback, the 4k Glass some feedback, and the S Rika feedback somewhat close to the Nubatama 4k and not as good as the 5k Nubatama, my favorite (but out of stock at the moment - a victim of it's own success, which is why I list the 4k Bamboo in first place).
The 8k Glassstone is capable of yielding a very precise geometry, so it is easy to go a bit higher for a strop compound. The 8k stone is about a 2 micron grit. So for a strop I'd suggest a 0.75 micron CBN compound, which is a 24,000 grit edge. This is perfect for bringing your edge easily to another level past 8k and still aggressive enough to be used for touchups. It is a 3x jump (8k * 3 = 24k). You will get your best edge with this compound on Kangaroo, but balsa or bovine leather is adequate. The results you get with this will far exceed using the Bark River Compounds, which are lower grits and less finely graded.
Hope this helps.