Question 146133
Assume the boat speed is Xmph
the river speed is 4mph

Since the operators asked him to turn back after 4 hours
  and return journey will take 6 hours
   (remember that the speed is added in one way, and reduced in return)

   (x+4) * 4 = (x-4) * 6
   4x+16     = 6x-24
   2x        = 40
    x        = 20

   speed of boat must be 20mph