THC can stay in the body for weeks. If you smoke everyday, then there is nothing you can do to get rid of the THC in 4 days. Some people say drink a lot of water, but that's a myth. THC is not water soluble so drinking a lot of water won't do anything. And no, you can't filter out the THC from the urine.
Only way I can think of is to bring some urine from someone who hasn't used drugs recently in a small baggie or cup and then use that instead of your own urine.
You'll need some way to heat it up to your body temperature. Maybe use one of those heat packs that people often put in their gloves when they go skiing to heat the urine?
