You do not want to transition from 4" to 3" as it will cause problems. You can only go from smaller to larger 3"-4" or 4"-5" ect. as you are heading to the street.
Also if the ash pit is active and the leaking water is passing through ashes the resulting liquid will be potassium hydroxide or potassium based lye. This can be very caustic and burn skin and eyes if contact is made. Flush with water if you get any on you.