I agree with what other monks have said, i.e. that it is probably not a very good idea to make things more complicated by trying to make them as compact as possible. However, for the sake of the exercise, this is possible one-liner (well, really a two-liner) to do what you want: 
perl  -F: -nale ' push @d, [@F]; END { print join "\n", map { join ":"
+, @$_} 
 grep { $c = $$_[1] eq $prec? $c+1:1; $prec = $$_[1]; $c>4? 0: 1} sort
+ {$a->[1] cmp $b->[1] || $b->[0] <=> $a->[0]} @d;};' file.txt

[download]
Note that it is also sorting the input by country in the event that the input is not grouped by country. The following is an example execution, piping your reshuffled input data (not grouped by country) into the Perl script: 
$ echo '20470:ZM:Samfya:Africa
> 61739:ZW:Chinhoyi:Africa
> 20149:ZM:Sesheke:Africa
> 26459:ZW:Beitbridge:Africa
> 37423:ZW:Bindura:Africa
> 18638:ZM:Siavonga:Africa
> 699385:ZW:Bulawayo:Africa
> 47294:ZW:Chegutu:Africa
> 18860:ZW:Chipinge:Africa
> 28205:ZW:Chiredzi:Africa
> ' | perl  -F: -nale ' push @d, [@F];
>  END { print join "\n", map { join ":", @$_}
>  grep { $c = $$_[1] eq $prec? $c+1:1; $prec = $$_[1]; $c>4? 0: 1}
>  sort {$a->[1] cmp $b->[1] || $b->[0] <=> $a->[0]} @d;}'


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

[download]
Edit 15:33 UTC: I posted my final result above only a few hours after I started to look for a solution because I had to interrupt my work on it for family obligations. I had not seen your solution using splice, it is better than my grep solution. Overall, I spent probably about 45 minutes to get it (hopefully) right, writing a solution with regular while or for loops would have probably taken less than a third of that time.

In reply to Re: Using map function to print few elements of list returned by sort function by Laurent_R
in thread Using map function to print few elements of list returned by sort function by jaypal

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