How to get your DS3/Poser runtimes into DS4:

The default for DS3 is to use the Content Manager database (Category view), rather than the Folder view which shows the files and folders as they are on the hard drive.  If you click the little triangle at the upper right of the Content tab in DS3 you can see which option is checked.

When you first run DS4 it should ask you if you want to "Import Mapped Folders" and if you want to "Migrate the Content Database".  You can access these options by right clicking on the tab that says "Content Library" in DS4.  

First "Import Mapped Folders".  If you then look at the Content Library tab you can click on "DAZ Studio Content Folders" or "Poser Content Folders" and see the folders as they are on disk, equivalent to the Folder view in DS3.

"Migrate the Content Database" isn't working properly for some people, but what it's supposed to do is import the Content Database used in the Category view in DS3.  After you do this, clicking on "Categories" in the Content Library tab in DS4 will give you the equivalent of the Category view in DS3.

According to this bug report, fixing the Migrate Content Database has immediate priority and has been assigned:
https://bugs.daz3d.com/view.php?id=42106