4. A particle moves on the x - axis starting at x=0. With equal probability, the particle moves to the left or right one unit. Let X denote the position on the x - axis after four moves. (a) Find pX(x). Notice that X can take on 4,2,0,2,4. Fill out the table below. (b) What is the expected position of the particle after four moves. That is, compute E(X). Express your answer as a fraction in lowest terms..