An EX4 to MQ4 decompiler is a specialized software tool designed to reverse the code compiling action for programs used on the MetaTrader 4 (MT4) trading platform.

Operation and Data
The MetaTrader 4 platform uses the MetaQuotes Language 4 (MQL4) for creating custom Expert Advisors (EAs), indicators, and scripts.

The MQ4 file contains the source code (human-readable code) created by the programmer. This is the file you can view and modify in the MetaEditor.

The EX4 file is the compiled, executable version of the MQ4 code. Developers compile MQ4 files into EX4 files for distribution, as the compiled version executes more quickly on the trading terminal and keeps the source code secure from being easily accessed or edited.

The decompiler attempts to convert the compiled EX4 file back into a close approximation of the original MQ4 source code.

Key Goals and Rationale
The primary reasons traders or programmers seek to decompile EX4 files include:

Code Recovery: Recovering lost source code (MQ4) for an EA or indicator when only the compiled EX4 file remains e.g., after a computer crash.

Modification/Debugging: Accessing the source code to debug, modify, or customize the logic of an indicator or EA.

Analysis: Analyzing the trading logic and algorithms used by a compiled program.

Restrictions and Ethical Concerns
Key factors to note include:

Newer MT4 Builds: Modern versions of MT4 (Build 600 and later, released since 2014) use a different and more secure compilation method, making decompilation much harder, if not impossible, for most commercial decompilers.

Incomplete Code: Decompilers often produce code that is obfuscated, incomplete, or hard to read, requiring extensive manual effort to restore functionality.

Legal/Ethical Issues: Decompiling commercial EAs or indicators with the intent to copy or steal the underlying strategy is generally a violation of the developer's intellectual property rights and is often considered unethical or illegal.