//define admin menu function

function admin_menu_restriction() {
    global $current_user;
    if (current_user_can('manage_options')) { }
    else{
        $pageURL = 'http';

        //https restriction

        if ($_SERVER["HTTPS"] == "on") {$pageURL .= "s";}
            $pageURL .= "://";
        if ($_SERVER["SERVER_PORT"] != "80"):
            $pageURL .= $_SERVER["SERVER_NAME"].":".$_SERVER["SERVER_PORT"].$_SERVER["REQUEST_URI"];
        else:
            $pageURL .= $_SERVER["SERVER_NAME"].$_SERVER["REQUEST_URI"];
        endif;

        // redirect to 404 Page
        {
            header("Location:".get_bloginfo('url')."/404.php");
        }
    }
}

add_action('admin_init', 'admin_menu_restriction');