If the common difference is 0, then we're adding 0 to each term to get the next term. However, adding 0 doesn't change the number. So that's why we have the same thing written four times.
As another example, let's say the common difference was 1 instead of 0. That would mean the first four terms are -2, -1, 0, 1 since we add 1 to each term to get the next one (eg: -2+1 = -1).
