I would say it's the borderlands expansion then. Try erasing it and seeing if you can play the DLC for it.
I wouldn't, because it is a single file, not 4 individual files, and it is not in the Borderlands folder like the DLC would be.
It says expansion pack. The borderlands goty expansion pack that he has already stated he has and downloaded is 4.6 gigs, the same as his mysterious folder. It's a pretty good bet. At the very least it's something worth trying.
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Borderlands does not have A expansion pack, it has 4 DLCs.
Content discs install the individual DLCs, not one large file that contains all the DLC.
The GotY install for Borderlands would list "Zombie Island" at 1014MB, "Mad Moxxi's Underdome Riot" at 512MB, "The Secret Armory of General Knoxx" at 1.7GB, and "Claptrap's New Robot Revolution" at 1.4GB, all within the Borderlands folder (names and files sizes taken from the listing on my HDD), it would not list a single file simply called "Expansion Pack" at 4.6GB as its own folder.
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