Using an EX4 decompiler typically violates MetaQuotes' license agreement, which states: “You may not reverse engineer, decompile, or disassemble the Software.”
Some professionals specialize in "lost source recovery." They will decompile the EX4 for you , and deliver a clean MQ4. Expect to pay $100–$500 depending on EA complexity.
The open-source community has done an impressive job reverse-engineering older builds of the MetaTrader 4 (MT4) platform. If you look hard enough, you will find repositories containing Python scripts (often utilizing pypi packages like mql-decompiler ) or older C++ binaries that can handle build 600-900 files quite well. For legacy EAs, these tools are lifesavers. The code is often open-source, allowing you to verify there are no backdoors or malware embedded in the decompiler itself—something you can never be sure of with "free decompiler" websites.
An EX4 decompiler is a tool designed to reverse-engineer compiled MQL4 machine code back into human-readable .mq4 source code.