#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#.