Regarding the information from IMDb that's not coming in complete - have you tried turning on the IMDb Options under the Plug-ins menu (next to the Help menu). There is an option to download the Complete Actors list.
Otherwise, please let us know the movie you're having trouble with so we can try to reproduce the problem here and fix it.
For the starring field in version 4: could you maybe use one of the new custom fields and name it 'Actors' (to differentiate it from Starring in the table view)? That way the field will be editable and you can enter the exact information that you want.
If that sounds like an option, here's how you can get your Starring info from the previous version into the new database:
-- Run DVDpedia 3.6 and export your library and any excluded collections in .dvdpedia format.
-- Select the exported file in the Finder, ctrl-click it and choose 'Show package contents' to see the .xml file inside.
-- Open that .xml file with any good text editor and do a find for <key>Starring</key> and replace it with <key>custom5</key> (or whichever custom field you'd prefer to use).
-- Now save the file and start up DVDpedia 4.0
-- Drag the .dvdpedia export file over the DVDpedia icon to import it.
The import will be placed in a library-independent collection so you can check that everything went well. If everything went okay, you can select all the entries in your Library collection, delete them, and then include the imported collection in the library. Now you just need to rename the Custom field you used for Starring to 'Actors' or 'Starring 2' or something like that.