my $string = '5424123412345678';
substr$_,0,-4,'*'x(length()-4)for$string;  # 41
print $string;
[download]

my $string = '5424123412345678';
s/(?!.{0,4}$)./*/g for$string;  # 30
print $string;
[download]

Update: More readable than the previous:

my $string = '5424123412345678';
$string=~s/(?!.{0,4}$)./*/g;  # 28
print $string;
[download]