Years ago, the MetaQuotes Corporation implemented significant updates to the MetaTrader 4 environment (specifically after Build 600). These updates drastically changed how files are compiled, introducing high-level encryption and security measures.
The most reliable way to obtain or update a source file is to contact the original author or the company that provided the software. ex4 to mq4 decompiler 40432 updated
The vast majority of these downloadable "decompilers" are actually disguised malware. Once executed, they can install keyloggers or remote access trojans (RATs) on your trading computer. ex4 to mq4 decompiler 40432 updated