This is the compiled version. When you finish writing an MQ4 file, the MetaEditor "compiles" it into an EX4 so the MT4 platform can run it. This version is machine-readable but human-unreadable.
Old decompilers used to work flawlessly on EX4 files. However, after Build 600+, MetaQuotes implemented much stronger encryption. When people search for "501 exe," they are often looking for the last version of the decompiler that was semi-functional before the major security overhaul. ex4 to mq4 decompiler 50 1 exe link