Downloading from the App Store the message "There was an error in the app store. Try again later (4)'. Then 'unexpected error'.

Question

Downloading from the App Store the message "There was an error in the app store.  Try again later (4)'. Then 'unexpected error'.
I've been getting this message now for the last couple of days.  I can download other apps on my phone with the same apple ID but have now experienced this on my Mac firstly when downloading Yosemite immediately after being asked for my ID password, and then having tested with a couple of other apps, so it's not related to the download.
I'm using OS X 10.9.5 on a Mac Book Air.
I've shut down on a number of occasions and tried again
App Store is version 1.3 (201.7)

Answer 1

Back up all data before proceeding.
Triple-click anywhere in the line below on this page to select it:
/Library/Preferences/SystemConfiguration/NetworkInterfaces.plist
Right-click or control-click the line and select
          Services ▹ Reveal in Finder (or just Reveal)
from the contextual menu.* A folder should open with an item selected. Move the selected item to the Trash. You may be prompted for your administrator login password. Restart the computer and test. If there's no change, restore the item you deleted from a backup, overwriting the one that may have been created in its place. Otherwise, you may need to recreate some of your settings in the Network preference pane.
*If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination command-C. In the Finder, select
          Go ▹ Go to Folder...
from the menu bar and paste into the box that opens by pressing command-V. You won't see what you pasted because a line break is included. Press return.
Credit for this solution to ASC member luis.pinho.