If you continue the line of the second step (from the top), you will get a parallel to the floor which will also equal to 4 feet.
Now the right triangle just created has one leg = 4 feet and another = 3 feet.
Calculate the hypotenuse (Railing) using Pythagoras;
(Railing)² = 4² + 3²
((Railing)² = 4² + 3²
Railing)² = 16 + 9 =25
(Railing)² = 25 and Railing = √25 = 5
