This is a question of permutation, where;
There is only one farmer who who has to plow four field. The farmer can choose to start and end with any field. Since ordering is critical, permutation theory comes into play.
Orders of plowing = nPr = n!/(n-r)!
Where
n = 4
r = 1
Substituting;
Orders of plowing = 4P1 = 4!/(4-1)! = 4
