Before answering this question, I would like to tell about the Harmonic Progression. The terms a, b, c are said to be in harmonic progression if 1/a, 1/b, and 1/c are in Arithmetic Progression, and we know that the difference of two consecutive terms of AP are constant and called as common difference. Here the four terms are given as a, b, c, and d. So 1/a, 1/b, 1/c, and 1/d would be AP and the difference between these consective terms would be same.
We can have the following relation with the above concept.
1/b - 1/a = 1/c - 1/b = 1/d - 1/c
or (a-b)/ab = (b-c)/bc = (c-d)/cd