Custom shortcode with do_shortcode and substr

using php substr requires you add a start point when calling substr function
http://php.net/manual/en/function.substr.php

A possible fix:

function my_shortcode_function() {
$string = "";
$string = do_shortcode('[different_shortcode]');
$string = substr($string, 0, 4);
return $string;
}

add_shortcode( 'my_shortcode', 'my_shortcode_function' );