To measure exactly 4 liters with a 5 liter and 3 liter bowl, fill the 5 liter bowl with water, pour it into the 3 liter bowl which will leave 2 liters in the 5 liter bowl, then pour the remaining 2 liters back into the 5 liter bowl, and repeat the process one more time to end up with 4 liters total between the two bowls.