input: Write a python function to create a Keras Sequential model with three hidden layers and dropout, as shown below.
  assert model.layers[0].units == 32
  assert model.layers[2].units == 16
  assert model.layers[4].units == 8
  assert model.layers[6].units == 4
output: def create_keras_model():
      from tensorflow.keras.models import Sequential
      from tensorflow.keras.layers import Dense, Dropout
  
      model = Sequential([
          Dense(32, activation='relu', input_shape=(100,)),
          Dropout(0.5),
          Dense(16, activation='relu'),
          Dropout(0.5),
          Dense(8, activation='relu'),
          Dropout(0.5),
          Dense(4, activation='relu')
      ])
  
      return model