How can I increase the WordPress image scaling threshold using PHP?

<?php
// Increase the image resize threshold to 4000px on the longest edge
function wpsc_smartwp_big_image_size_threshold( $threshold ) {
 return 4000;
}
add_filter( 'big_image_size_threshold', 'wpsc_smartwp_big_image_size_threshold', 999, 1);