The mean deviation of a set of numbers is the average of the absolute differences between each number and the mean of the set.

For the given set of numbers {1, 2, 3, 4}, the mean is (1 + 2 + 3 + 4) / 4 = 2.5.

The absolute differences between each number and the mean are |1 - 2.5| = 1.5,

|2 - 2.5| = 0.5,

|3 - 2.5| = 0.5, and

|4 - 2.5| = 1.5.

The mean deviation is the average of these absolute differences, which is (1.5 + 0.5 + 0.5 + 1.5) / 4 = 1.