(
  ZM => [ [20470, "20470:ZM:Samfya:Africa"],
          [20149, "20149:ZM:Sesheke:Africa"],
          [18638, "18638:ZM:Siavonga:Africa"] ],
  ZW => [ ... ],
  ...
)

##</code><code>##

use warnings;       # just for debugging
use strict;         # just for debugging
my (%h, $k, @F);    # just for debugging

while (<>) {        # -n
  chomp;            # -l
  $\ = "\n";        # -l
  @F = split(':');  # -F":" -a
  
  push @{$h{ $F[1] }}, [$F[0], $_];
}
for $k (sort keys %h) {
  print $_->[1] for splice [sort {$b->[0]<=>$a->[0]} @{$h{$k}}], 0, 4
}                   # -n