How wonderful ! And another regex trick learned :-)

Though it goes bloomin' berserk backtrackin' when all four of 'abcd' are not present ! The following tweak improves:

    my @in_order = ($str =~ m{ 
       ^(?> .*?              ([abcd]) )
        (?> .*? (?!\1)       ([abcd]) )
        (?> .*? (?!\1|\2)    ([abcd]) )
        (?> .*? (?!\1|\2|\3) ([abcd]) )
    }x);
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And, of course, YAW:

  sub four_or_less {
    my ($s) = @_ ;

    $s =~ tr/abcd//cd ;

    my $c = '' ;
    $c .= $1 while ($s =~ m/([^ $c])/g)  ;

    return $c ;
  }
[download]