#!/usr/local/bin/perl

use strict;
use warnings;
use Data::Dumper;

my (%HoA, %lines);

while (my $line = <DATA>) {
    my ($value, $key) = split /:/, $line, 3;
    push @{$HoA{$key}}, $value;
    $lines{"$key $value"} = $line          # This could be done better
}

for my $country (keys %HoA) {
    my @list = sort { $b <=> $a } @{$HoA{$country}};  # This could be done better
    for my $ind (0 .. 3) {    # This could be done better
        my $popu = $list[$ind] or next;
        print $lines{"$country $popu"};
    }
}

__DATA__
20470:ZM:Samfya:Africa
20149:ZM:Sesheke:Africa
18638:ZM:Siavonga:Africa
26459:ZW:Beitbridge:Africa
37423:ZW:Bindura:Africa
699385:ZW:Bulawayo:Africa
47294:ZW:Chegutu:Africa
61739:ZW:Chinhoyi:Africa
18860:ZW:Chipinge:Africa
28205:ZW:Chiredzi:Africa