The most reliable and legal method to get an editable .mq4 file is to reach out to the original creator or vendor. Many developers are willing to provide the source code for a reasonable fee or under a specialized licensing agreement.
The latest update of the Ex4 to Mq4 Decompiler, version 4.0.432, includes several key improvements: Ex4 To Mq4 Decompiler 4.0.432 - Updated