The 4th Office allows you to lock documents while you're working on them, to avoid others making changes at the same time which may then lead to version confusion.
Automatic document locking:
When you open documents using the 4th Office Edit you will be informed about locking the document if you wish.
The header on the document preview will indicate the document is locked. Once you have finished working on the document and saved it back to the 4th Office you can unlock it.
The document thumbnails will also show you if the document is locked, and give you the option to unlock it.