The outer dimensions of the garden surrounded by the walkway are 20+2*4 = 28 ft by 30+2*4 = 38 ft.
Therefore, the area of this plot of ground is 28*38 square feet.
The area of the garden itself is 20*30 square feet.
Thus the area of the walkway is the difference
28*38 - 20*30 = 464 square feet. ANSWER