the number three plays a big role in fairy tales, but when I count it is generally four... the fourth is not just another additional number unit; it is not another thing of the same kind, but something completely different.
Von Franz argues that the apparent primacy of three in fairy tales is structurally subordinate to a four-term rhythm, in which three stages of similar repetition are followed by a qualitatively different fourth — making three a marker of process rather than completion.
, The Interpretation of Fairy Tales, 1970thesis