If a virus is found and you select the action "Move" from the "Virus found!" dialog, the file is moved into the directory Data\Moved and a .vir extension is appended (at least that's the default setting). So, if you have any files in there, you have probably instructed avast! to move there some infected files it found.
I suppose you can delete it (even though 4GB is certainly very big... what could it be?)