Use the EX4’s behavior to observe in strategy tester. Document entry rules, exit logic, risk management. Then code a new version. This is legal and improves your skills.
If you own the rights to an EX4 and lost the MQ4, the safest path is to rewrite the logic from scratch or consult a professional MQL4 programmer. If you are trying to crack a commercial EA – remember that doing so harms the trading ecosystem and violates laws.
The decompiler is primarily effective against EX4 files compiled with MT4 build 509 or lower (versions released in 2013 or earlier).
The original, editable code. It contains logic, parameters, and comments. Losing the .mq4 while retaining the .ex4 is common – due to hard drive failures, accidental deletion, or purchasing a black-box EA from a vendor.