I cannot sign into App Store. "unknown error" and then "We cannot complete your request. There has been an error in the App Store. Please try again later. (4)"

Does the flag in the lower right corner of the App Store window match the country of your billing address?

I also have the same problem - starting two days ago, but with a purchased app:

When I attempt to download the app, it says "unknown error" and then it says "We cannot complete your request. There has been an error in the App Store. Please try again later. (4)"

That sounds like a problem with your account. You would have to contact App Store Support for a resolution.

You could try this.

Back up all data.

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

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. Reboot 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, paste into the box that opens (command-V). You won't see what you pasted because a line break is included. Press return.