From the condition, you have this equation


 = ,


where U is the speed of the boat in still water.


Notice that each side of the equation is the corresponding traveled time.


Reduce the common factor 28 in both sides; then cross multiply.  You will get


    2*(u-4) = u + 4

    2u - 8 = u + 4

    2u - u = 4 + 8

     u     = 12.


ANSWER.  The speed of the boat is 12 miles per hour in still water.