You will often see "Version 4.0.5" or "V4" advertised on various forums and file-sharing sites. Historically, older versions of MT4 (prior to Build 600) were relatively easy to decompile. However, MetaQuotes (the developer of MT4) implemented a massive security overhaul years ago.
Before searching for a "full exclusive" download, consider the following risks:
Are you trying to or are you looking to modify an existing indicator ?