Right-click the Library icon in the toolbar.
There are two (pertinent, and several irrelevant) menu options:
- Switch/Create Library.
Use this to create new libraries, but one of the options in the popup allows you to add a pre-existing library from the specified location.
- Export/Import all calibre data.
Brand-new tool for migrating to another computer. If you weren't backed up to Dropbox, you would use this to save your settings + all four libraries, then import the save folder on the new computer to restore everything.