4) C

Total number of balls = (x + y + z)

Probability of first ball being red  =          x        
                                                           x + y + z

Since the ball is replace, the total number of balls will remain the same, i.e., (x + y + z)

Probability of second ball being blue =         y        
                                                                x + y + z

Probability of first ball being red and second ball being blue:

     x          x          y       
x + y + z         x + y + z

        xy      
 (x + y + z)²