If you have lost the original MQ4 file but still have a copy of the EX4 file you authored, you might be able to use a decompiler to recover a working version. This is typically the one legitimate scenario for using such tools.
Always ensure you have the explicit permission of the original developer before attempting to decompile an EX4 file. Safe Alternatives to Decompilation
However, the world of MQL4 decompilation is filled with broken tools, security risks, and technical dead ends. Here is what you need to know before you download that "decompiler.zip" from a random repository. 1. Can You Actually Decompile EX4 Files in 2026?