I'm not sure what you mean by being on the same cog, do you mean 3rd and 4th gears share the same cog? If so I believe those are separate cogs but what they do share is their synchro hub. If I'm not mistaken 3rd gear has its own synchro ring on one side of the hub and 4th gear has its own synchro ring on the other side of the hub. So if you would have a worn 3rd gear synchro ring then 3rd gear would grind but 4th wouldn't as it has a different ring. Does it grind if you shift slower or at low rpm? What about if you're double clutching it?