Changing the "open with" application should make no difference to how MT4 operates, it only changes what application to use when you double click the file in Windows Explorer. By default ex4 files have no predefined application in Windows Explorer.
If you changed the actual file extensions from .ex4 to something else you will have to manually change them back again.
Thank you guys for your helpful answers
Hi my friends,
I've made a mistake and change the extension of .ex4 files through its properties and by " open with " button, I mean I chose to open .ex4 files in "notepad" application and it is completely crashed right now.
Does anyone know how to change it to its default condition in order to open in its predefined application, which I don't know what is ?
thanking in advance,

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Hi my friends,
I've made a mistake and change the extension of .ex4 files through its properties and by " open with " button, I mean I chose to open .ex4 files in "notepad" application and it is completely crashed right now.
Does anyone know how to change it to its default condition in order to open in its predefined application, which I don't know what is ?
thanking in advance,