In the video, " Dropout Regularization, inverted dropout", teacher said " So, in order to not reduce the expected value of z^4, what you do is you need to take this, and divide it by 0.8 because this will correct or just a bump that back up by roughly 20% that you need."

So. my question is why we need to keep the E[Z^4] ? If E[Z^4] is reduced from the a3, what will happen??