Hello,
Unfortunately, we haven't tested urlrewriter.net with Kentico CMS, but you can eventually try it.
You could also add old urls as aliases for documents in Kentico CMS 4.0 or ensure redirection (e.g. 301 permanent redirect) by custom code in Application_BeginRequest method in ~\App_Code\Global.asax.cs file. Please see
this thread for more details.
Best Regards,
Martin Dobsicek