4 ss is the class of ss steel this will not rust but in time will turn orange you can check it with a magnet it will draw to it is magnetic and is used in exhaust and 3 class is not magentic and is use in the medical field and used in food plants and pipes pans tanks any thing that will not rust and can take corrsion there is many thing s it used in but this is the short list if you have class 3