The App Store won't even let me download Yosemite.

Question

All I get is this message: "We could not complete your request. There was an error in the App Store. Please try again later. (4)"
and then "UNKNOWN ERROR"

Answer 1

Launch the Console application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad and start typing the name.
Select
          /var/log ▹ install.log
from the log list on the left. If you don't see that list, select
          View ▹ Show Log List
from the menu bar at the top of the screen. The contents of the log will appear on the right. Each log message begins with a timestamp. Select the messages from the time of the last installation or update attempt. If you're not sure when that was, click the Clear Display button in the toolbar of the Console window and then try the installation again. Select the new messages that appear. Copy the messages to the Clipboard by pressing the key combination command-C. Paste into a reply to this message (command-V).
If there are runs of repeated messages, post only one example of each. Don’t post many repetitions of the same message.
When posting a log extract, be selective. Don't post more than is requested.
Please don't indiscriminately dump thousands of lines from the log into this discussion.
Please don't post screenshots of log messages—post the text.
Some private information, such as your name, may appear in the log. Anonymize before posting. You may need to use a text editor with search and replace, such as TextEdit.
When you post the log extract, you might see an error message on the web page: "You have included content in your post that is not permitted," or "The message contains invalid characters." That's a bug in the forum software. Please post the text on Pastebin, then post a link here to the page you created.