use Algorithm::Bucketizer;
while (my $in = <>) {
    chomp($in);
    my %counts;
    $counts{$_}++ for split //, $in;
    my $b = Algorithm::Bucketizer->new(bucketsize=>length($in)/4);
    $b->add_item($_, $counts{$_}) for keys %counts;
    $b->optimize(maxrounds=>100);
    print $_->serial, " => ",
        join(", ", map {"$_ ($counts{$_})"} $_->items ),
        "\n" for $b->buckets;
}

##</code><code>##

chhbfbjjjcgaiiaachbaefeabcbjffdgaggbgccaihgjddedfheejdigcdeiacebdehfigegifibdabhfdihbdhifcgjfjhcaejj
1 => f (10), g (10)
2 => b (10), h (10)
3 => i (10), j (10)
4 => e (10), d (10)
5 => c (10), a (10)
abbbbbbbccddddddeeefffffgggghhhh
1 => e (3), f (5)
2 => b (7), a (1)
3 => c (2), d (6)
4 => g (4), h (4)