After you upgrade from DAZ Studio 3 to 4, you may no longer find your previous content. You can import mapped directories so that your previously installed content is now visible in DAZ Studio 4.
Import Mapped Directories
https://helpdaz.zendesk.com/entries/20643848-how-to-bring-existing-content-into-daz-studio-4
You can also map specific folders. To do this, go to the Content Directory Manager. Here's how to get there. Go to EDIT (Win) --> Preferences or DAZStudio (Mac) --> Preferences. In Preferences, go to Content Library Tab and then click on the 'Content Directory Manager' button. In the new Window, look for 'Content Sets.' You will be able to expand and see the mapped directories for DAZ Studio Formats and Poser Formats. Keep in mind that you should never add the 'runtime' folder itself.
ARCHIVED (DAZ Studio 2.x and 3.x)
The reason that you cannot find your installed content is because Daz Studio loads the new Content Manager instead of the normal Content display. The Content Manager lets you customize how you would like to arrange and sort your content any way that you like it.
To find your missing content you will need to change the content display back to the original file structure from your hard drive. To do this please click on the Content Tab Options button that is located in the top right corner of the Content window and select "View Folders as Tree".