I don't let them get that bad. Generally I'll refresh on whatever the blade was finished on (either boron carbide for the sheffields or chrome oxide for most everything else). If that doesn't bring the blade back within 5 laps or so then I'll drop back one grit and try again. I don't think I've ever had to go back to the 4k unless I've done something stupid, the farthest I've ever had to go back was to the arkansas or 8k.