Text. Let G be a finite cyclic group. Every sequence S over G can be written in the form S = (n 1 g) · . . . · (n l g) where g ∈ G and n 1 , . . . ,n l ∈ [1, ord(g)], and the index ind(S) of S is defined to be the minimum of (n 1 + · · · + n l )/ord(g) over all possible g ∈ G such that g = supp(S) . The problem regarding the index of sequences has been studied in a series of papers, and a main focus is to determine sequences of index 1. In the present paper, we show that if G is a cyclic of prime power order such that gcd(|G|, 6) = 1, then every minimal zero-sum sequence of length 4 has index 1.Video. For a video summary of this paper, please click here or visit http://www.youtube.com/watch?v=BC7josX_xVs.