If you don't let the edge degrade too much the 12k would do it. I haven't used 10k.
Of course, the question is what is 'too long' and I can't answer it over the internet. After honing many razors I have an idea what hone is appropriate when I have a particular razor, but that's not your position. The good thing about the norton combo 4k/8k hone is that it's two hones that can sharpen any undamaged previously shaving razor with less than 20 strokes.