The plugin generated x characters of unexpected output

I'm writing WordPress plugin but a error.

Error : The plugin generated 4 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.

<?php
/*
Plugin Name: XXXXX
Plugin URI: XXXXX
Description: XXXXX
Version: 1.0
Author: XXXXX
Author URI: XXXXX
License: GNU
*/
echo"test";
?>

via Chebli Mohamed