Before trying to decompile, check the MQL5 Market or GitHub. Many popular EAs have open-source versions or similar logic available for free. Final Word on Ethics and Legality
Years ago, MetaQuotes (the developers of MT4) released a massive update (Build 600+) that changed the way EX4 files are compiled. This update implemented high-level encryption and a completely different file structure. Ex4 To Mq4 V4 Decompiler Version 4.0 5 Full Version
There are professional services that perform "manual decompilation." Instead of using a one-click tool, experts use debuggers to reconstruct the code logic. These are expensive but are the only way to handle modern EX4 encryption. Before trying to decompile, check the MQL5 Market or GitHub