We'll first find the area of the top of the walkway in square feet
and then multiply that area by the the thickness of 4 inches, or
 ft to find the volume.  We'll find it in cubic feet and 
then convert to cubic yards.

The inner rectangle is 20ft×35ft, so it has an area of 700 sq.ft.  

The larger rectangle is 6 feet more on each dimension (since the
walkway is 3 feet more on each side of each dimension), so it is
26ft×41ft.  It has an area of 1066 sq. ft. This is the picture of 
the top of the walk. 

 

To find the area of the top of the walkway, we subtract the area
of the inner rectangle from the area of the larger rectangle.

Area of top of walkway = 1066 sq ft - 700 sq ft = 366 sq ft.

Since it is 4 inches thick, which is  ft, we find the 
volume by multiplying the area of the top of the walkway by the
thickness and get 366× = 122 cubic feet.

Finally we must convert cubic feet to cubic yards.  There are 27 cubic
feet in 1 cubic yard, so we divide 122 by 27 and get 4.518518519
cubic yards.

Edwin