Fallout 4's Far Harbour DLC arrived a few weeks ago, but sadly the PS4 version was hampered by poor performance.
Thankfully Bethesda has now figured out the DLC's problems and has released an updated version of Far Harbour which "addresses performance issues". This version is currently only available on the US PlayStation Store, with Bethesda promising info on the EU update in the very near future.
Here's what you need to do in order to get the updated version of Far Harbour.
1. Confirm Fallout 4 is not running. Highlight Fallout 4 in your PS4 dashboard, press OPTIONS, and select CLOSE APPLICATION to ensure Fallout 4 is not running minimized.
2. Select RELATED ITEMS. You’ll find this under the Fallout 4 icon in the PS4 dashboard.
3. Select MY ADD-ONS.
4. Navigate to the DOWNLOAD ARROW next to Far Harbor and select it.
5. You should see a notice that Far Harbor has been Added to Downloads.
6. Wait for the downloadable content to finish downloading and installing before starting Fallout 4.
If this doesn't work, Bethesda's support page suggests deleting Fallout 4 and all add-on content and then reinstalling the game from scratch.