The "ex4 to mq4 decompiler 50 1 exe" tool is likely one of the several tools available online that claim to decompile .ex4 files back into .mq4 source code. When searching for and using such tools:
Before diving into using any decompiler: ex4 to mq4 decompiler 50 1 exe
File name example: ex4_to_mq4_decompiler_50_1.exe
Claimed purpose: Convert compiled MetaTrader 4 executable files (.ex4) back into human-readable MQL4 source code (.mq4).
Version identifier: 50.1 (suggesting multiple iterations, possibly targeting build 509+ of MT4). The "ex4 to mq4 decompiler 50 1 exe"
Most legitimate decompilation attempts face serious limitations: Decompiling copyrighted software to reverse-engineer
This is a gray area. If you wrote the code yourself and lost the source file, you have the right to decompile your own work. However, most EX4 files found online are copyrighted by developers who sell them.
Decompiling copyrighted software to reverse-engineer, modify, or redistribute it without permission is a violation of copyright law and intellectual property rights.