If you are using 4D Insider to open a structure file and receive a message stating that the file is "locked", this means that the file is set to Read-Only mode.
To unlock it:
Right-click on the structure file in the Windows Explorer and select "Properties". A dialog box will open where the Read-Only option is selected.
De-select the Read-Only property, click Apply or close the window, and you will now be able to open the structure file with 4D Insider.