They can both use the same content—I highly recommend having DS4.5 read DS3’s content folders. The application folder itself is pretty small compared to the content—a couple of hundred MB at most for the application folder, a lot of content these days has 50-100MB of textures for one product.
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 from the Content Library options menu if it doesn’t prompt you automatically.
It should also ask if you want to import metadata. If you want to put the default content in your own categories, uncheck them—then the default content will all appear in Default > Unassigned.
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.
Then “Migrate the Content Database” to 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, plus an extra top-level category “Default” for the DS4 content. You can now categorize the DS4 content from Default > Unassigned into your own categories.
[Note: Migrating will skip items which have non-English characters, such as diacritical marks.]
Now go to the Content Library options menu, “DB Maintenance > Re-import metadata” to apply the metadata used in the Smart Content tab. This will add the content to the Default categories, but it will also leave it in your own categories. If it is now also in the Default categories and you’d prefer it wasn’t, go into Default and right-click on each subcategory to delete it. The categories will come back when you next start DS, but they will be empty.