php - Regular expression to replace digits after first 4 digits -

i trying write regular expression replaces digits of number *'s after first 4 digits.

for example

var number = 123456789   

it should replaced 1234*****

in javascript:

var maskednumber = string(number).substr(0,4) + array(string(number).length - 3).join('*'); 

in php:

$maskednumber = str_pad(substr($number, 0, 4), strlen($number), "*");