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<?php
add_action( 'pre_get_posts', 'be_exclude_category_from_blog' );
/**
 * Exclude Category from Blog
 * 
 * @author Bill Erickson
 * @link http://www.billerickson.net/customize-the-wordpress-query/
 * @param object $query data
 *
 */
function be_exclude_category_from_blog( $query ) {
	
	global $wp_the_query;
	if( $wp_the_query === $query && $query->is_home() ) {
		$query->set( 'cat', '-4' );
	}
}