Expedition 4 is a ruined town built and abandoned by the Tech Hunters. There are lots of items on ground in and around the ruined buildings. Unlike most ruins, this location actually has a working wind generator.
This location is referenced in Tech Hunter's Notes: The Ashlands. In many ways, it is similar to the Expedition 5 location. Although the number implicates that there were 3 expedition before it, it is however, not (at least explicitly) stated in the game what the results of those expeditions were.