A tank initially contains 4 gallons of pure water. A salt solution containing 1/2 lb. of salt per gallon is then poured into the tank at 4 gallons per minute, while the well mixed solution is simultaneously drained at 3 gallons per minute. Find the amount of salt in the tank after 4 minutes. (Simplify your final answer, it should be given in the form a/b where a and b are positive integers).