$ perl -le'
my $string = q/5424123412345678/;
$string =~ s/.(?=.{4})/*/g;                   
print $string;'
************5678

Update:

$ perl -le'
my $string = q/5424123412345678/;
$string =~ s/(?<=.{4}).(?=.{4})/*/g;
print $string;'
5424********5678
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