perl -F':' -lane '
push @{$h{$F[1]}}, [$F[0],$_] }{
print $_->[1] for map { splice [sort {$b->[0] <=> $a->[0]} @{$h{$_}}],0,4 } keys %h' 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