Update H4

exercises/h4.md

@@ -411,6 +411,8 @@ Header unset Vary
 Header unset ETag
 ```
 
+**NOTE:** `Header unset` settings may cause errors on starting up the server. For example, you may consider usage of `FileETag None` as a replacement for `Header unset ETag`.
+
 **NOTE:** You must be careful when unsetting fields because it affects behavior of client programs and efficiency of your server environment (performance, for instance). Remember that factors such as field order, formatting and error messages can give hints about the used server environment as well (for instance, 404 not found message). 
 
 **NOTE:** The header options mentioned above work only if module `headers` has been activated (run command `sudo a2enmod headers` and restart the Apache server)