Years ago, older versions of MetaTrader 4 (specifically builds prior to build 600) used a simpler compilation method. During that era, simple automated tools called "decompilers" could easily reconstruct the MQ4 source code.
An EX4 file contains the same logic as the original MQ4, but in a different language—machine-like bytecode. Converting back is not like renaming a file; it is closer to translating French into English by someone who only knows the sounds, not the vocabulary.