求助一道奥数题答案

答法可能是
AAAA AAAB AAAC AAAD AABA AABB AABC AABD AACA AACB AACC AACD AADA AADB AADC AADD ABAA ABAB ABAC ABAD ABBA ABBB ABBC ABBD ABCA ABCB ABCC ABCD ABDA ABDB ABDC ABDD ACAA ACAB ACAC ACAD ACBA ACBB ACBC ACBD ACCA ACCB ACCC ACCD ACDA ACDB ACDC ACDD ADAA ADAB ADAC ADAD ADBA ADBB ADBC ADBD ADCA ADCB ADCC ADCD ADDA ADDB ADDC ADDD BAAA BAAB BAAC BAAD BABA BABB BABC BABD BACA BACB BACC BACD BADA BADB BADC BADD BBAA BBAB BBAC BBAD BBBA BBBB BBBC BBBD BBCA BBCB BBCC BBCD BBDA BBDB BBDC BBDD BCAA BCAB BCAC BCAD BCBA BCBB BCBC BCBD BCCA BCCB BCCC BCCD BCDA BCDB BCDC BCDD BDAA BDAB BDAC BDAD BDBA BDBB BDBC BDBD BDCA BDCB BDCC BDCD BDDA BDDB BDDC BDDD CAAA CAAB CAAC CAAD CABA CABB CABC CABD CACA CACB CACC CACD CADA CADB CADC CADD CBAA CCAB CBAC CBAD CBBA CBBB CBBC CBBD CBCA CBCB CBCC CBCD CBDA CBDB CBDC CBDD CCAA CCAB CCAC CCAD CCBA CCBB CCBC CCBD CCCA CCCB CCCC CCCD CCDA CCDB CCDC CCDD CDAA CDAB CDAC CDAD CDBA CDBB CDBC CDBD CDCA CDCB CDCC CDCD CDDA CDDB CDDC CDDD DAAA DAAB D