1. I think the 4K is plenty fine for most people and you don't need anything finer than that. A 4K edge will shave once you get the hang of the machine. If you want to add one more item I would suggest the 8K stone. You can use that for a final polish on your edges or for quick touch ups. I prefer to free hand strop my knives since it's quick and easy so opt for the 8K over a strop until you get the hang of the machine or don't add anything to the set. The only other item I think would really help you is a flattening plate. I like this one very much for flattening all my stones: http://www.chefknivestogo.com/at14dipl.html
2. You shouldn't need the skinny stones for your Wusthof Ikon steak knives. They have a tapered bolster with no finger guard so they're pretty simply to sharpen.
Good luck! If you have other questions once you use the machine please come back here and ask us. We'll be more than happy to help you.