Because the demand for these tools is high, hackers often bundle trojans or keyloggers into "cracked" decompiler software.
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. ex4tomq4 v4 decompiler version 405 full exclusive
A decompiler is a tool designed to reverse this process, attempting to turn the compiled binary back into human-readable code. The Myth of Version 4.0.5 "Full Exclusive" Because the demand for these tools is high,