Files with the "mq4" extension are EA source code files. While these files can also be stored in the folder, they cannot be used unless converted into an "ex4" file by a process called "compilation". Double-clicking on the file name will automatically launch the MetaEditor, an editing software for MQ4 files, and then you can compile the file.

Note that if you put the MQ4 file in the folder as it is and click the "Refresh" button as in step 5 on this page, the compilation will be performed automatically. However, with this method, any error messages will not be shown in case the compilation fails, so we recommend using MetaEditor to compile the file.