I believe that L is in feet while t is in seconds. To find for the hang time, all we have to do is to plug in the value of L in feet in the equation.
4 inches is equivalent to 4/12 feet or 0.33 feet, therefore the total L is:
L = 4.33 ft
Using the equation to find for t:
L = 4 t^2
4.33 = 4 t^2
t = 1.04 s