I really don't think the appeal will be successful. I don't think the point is whether he hit Marsh with his spit, or how far away Marsh is, or whether one leg is a little more behind the other one. It's more to do with the action of spitting and the intent therein. To me it seems clear that he spat in a offensive manner towards Marsh, regardless of whether he was trying to hit him or not. His action was intimidatory and deliberate. The argument that players spit all the time on the feild doesn't wash, sure everyone who plays sport probably spits plenty of times, but they choose to do it in a non-offensive, non-confrontational way, i.e in the back play.
Dunley knew what he was doing, he had time to give Marsh a verbal spray than to let loose with his low act. He deserves the 4 weeks. End of story.