given = [ 'ABCD';'CABD';'ACDB';'DACB'; ...
          'BCDA';'ACBD';'ADCB';'CDAB'; ...
          'DABC';'BCAD';'CADB';'CDBA'; ...
          'CBAD';'ABDC';'ADBC';'BDCA'; ...
          'DCBA';'BACD';'BADC';'BDAC'; ...
          'CBDA';'DBCA';'DCAB' ];
val = 4.^(3:-1:0)';
there = 1+(toascii(given)-toascii('A'))*val;
every = 1+perms(0:3)*val;

bits = zeros(max(every),1);
bits(every) = 1;
bits(there) = 0;
missing = dec2base(find(bits)-1,'ABCD')