D = Vt, where D is the distance/displacement, V is the speed of the wave, and t is the time it takes for the wave to travel across the field. Rearranging the variables in this equation to solve for V: V=D/t.
D = 40 m, and t = 4 *60 = 240 s (You want SI units). Therfore, V = 0.167 m/s.