use strict;
use warnings;

use Data::Dumper qw( Dumper );

my $hash = {
   k1 => [5, 'b1', 'c1', 1],
   k2 => [4, 'b2', 'c2', 1],
   k3 => [4, 'b3', 'c3', 3],
   k4 => [5, 'b4', 'c4', 3],
};

my $d_group;
my $a_group;
my $prio;
for my $rec (
    sort { $a->[3] <=> $b->[3] || $b->[0] <=> $a->[0] }
     values(%$hash)
) {
   if (!defined($d_group) || $rec->[3] != $d_group) {
      $d_group = $rec->[3];
      $a_group = $rec->[0];
      $prio = 1;
   }
   elsif ($rec->[0] != $a_group) {
      ++$prio;
   }

   $rec->[4] = $prio;
}

local $Data::Dumper::Sortkeys = 1;
print(Dumper($hash));
[download]

$VAR1 = {
          'k1' => [
                    5,
                    'b1',
                    'c1',
                    1,
                    1
                  ],
          'k2' => [
                    4,
                    'b2',
                    'c2',
                    1,
                    2
                  ],
          'k3' => [
                    4,
                    'b3',
                    'c3',
                    3,
                    2
                  ],
          'k4' => [
                    5,
                    'b4',
                    'c4',
                    3,
                    1
                  ]
        };
[download]

Update: Added test code. Tested. Fixed bug in sort expression.