0x20000004 is difficult to debug.
1) If your subscription allows the update right now (i.e., if it is not expired). Home (free) version need registering each 14 months.
2) If your firewall settings and allow
avast.setup to connect.
If you're not sure, just turn off your firewall (to be sure the update failure is not due to it).
Maybe you should 'remove' avast.setup entry, try to update, receiving a new alert from the firewall, allowing it and checking it to remember.
3) Check your proxy settings into avast!:
Left click the avast icon > Settings
Update (Basic) > Details > Proxy
Make sure your browser is not set to "Work Offline" (this option is generally in the File menu).
If it doesn't help, try switching from "Auto-detect" to "No proxy" (I'm assuming you're not connecting to the Internet via proxy).
4) Also you can try changing update service's login account, follow this:
Open services control panel (e.g. Start->Run, and type:
services.msc Double-click on avast! iAVS4 Control Service to show properties. Switch to LogOn page.
Enter administrator account name and password.
Do the same for avast! Antivirus service.
Then start update again.
5) The last option, check if the servers.def file is corrupt or 0KB (C:\Program Files\Alwil Software\Avast4\Setup\servers.def).
You can try and download the servers.def file,
http://files.avast.com/iavs4x/servers.def and replace the one in the avast setup folder.
