perl -F":" -lane '
BEGIN { $"=":" }
push @{$h{$F[1]}}, $F[0];
$line{$F[1],$F[0]} = "@F"; }{
for $k (keys %h) { print $line{$k,$_} for splice [sort { $b <=> $a } @{$h{$k}}] , 0, 4 }' file
20470:ZM:Samfya:Africa
20149:ZM:Sesheke:Africa
18638:ZM:Siavonga:Africa
699385:ZW:Bulawayo:Africa
61739:ZW:Chinhoyi:Africa
47294:ZW:Chegutu:Africa
37423:ZW:Bindura:Africa