1. Fill the 3 gallon bucket with water.

2. Pour the water from the 3 gallon bucket into the 5 gallon bucket.

3. Fill the 3 gallon bucket again with water.

4. Pour the water from the 3 gallon bucket into the 5 gallon bucket until it is full, leaving 1 gallon of water in the 3 gallon bucket.

5. Pour out the water from the 5 gallon bucket until it is empty.

6. Pour the 1 gallon of water from the 3 gallon bucket into the 5 gallon bucket.

7. Fill the 3 gallon bucket with water.

8. Pour the water from the 3 gallon bucket into the 5 gallon bucket until it is full, which will result in exactly 4 gallons of water in the 5 gallon bucket.