use Data::Dumper;
my $prev_data;
while (my $data = $sth->fetchrow_arrayref()) {
  if( $prev_data
    && $data->[0] == $prev_data->[0]
    && $data->[1] == $prev_data->[1]
    && $data->[2] == $prev_data->[2]
  ){
    print $data->[4];
  }else{
    print Dumper $data;
  }
  $prev_data = $data;
}

##</code><code>##

SELECT t.col0, t.col1, t.col2, t.col4
FROM mytable as t
  JOIN (
    SELECT col0, col1, col2, min(col4)
    FROM mytable
    GROUP BY col0, col1, col2
    HAVING COUNT(*) > 1
  ) as tmp ON
    tmp.col0 = t.col0
    AND tmp.col1 = t.col1
    AND tmp.col2 = t.col2
    AND tmp.col4 <> t.col4