functions.php

add_action( 'get_header', 'sk_set_404_layout' );
/**
 * Force full width content on 404 pages
 *
 * @author Sridhar Katakam
 * @link   http://sridharkatakam.com/
 */
function sk_set_404_layout() {
	if ( ! is_404() ) {
		return;
	}

	add_filter( 'genesis_pre_get_option_site_layout', '__genesis_return_full_width_content' );
}