The string with three each of four letters (ABCDABCDABCD) can be solved in four steps; the other three options each take five steps.
A[BCDABCDA]BCD > A[ADCBADCB]BCD
AA[DCBA]DCBBCD > AA[ABCD]DCBBCD
AAAB[CDDCBB]CD > AAAB[BBCDDC]CD
AAABBBC[DDCC]D > AAABBBC[CCDD]D