Decompilers are tools specifically designed to reverse-engineer compiled code back into a higher-level programming language. For MT4, there are a few decompilers available:

This is the source code file. It contains human-readable programming language written by a developer. You can open, view, and edit this file using the MetaEditor tool built into MetaTrader 4.

The conversion is almost perfect. For new builds: You need a more advanced tool.