Solution: The area of an equilateral triangle is given by $ \frac\sqrt34 s^2 $, where $ s $ is the side length. Setting $ \frac\sqrt34 s^2 = 36\sqrt3 $, we solve for $ s^2 = 144 $, so $ s = 12 \, \textcm $. The new side length is $ 12 - 4 = 8 \, \textcm $. The new area is $ \frac\sqrt34 \times 8^2 = 16\sqrt3 \, \textcm^2 $. The decrease in area is $ 36\sqrt3 - 16\sqrt3 = 20\sqrt3 \, \textcm^2 $. \boxed20\sqrt3 - Portal da Acústica