Error: Wallet backup failed! (code -4) when setting up bitcoin-qt for first time
I've looked at Bitcoin's backupwallet code, and that error is triggered by a filesystem error. I can't tell what kind of filesystem error it is, though. Please open debug.log in the Bitcoin directory. There should be a line like

&lt;date + time&gt; error copying wallet.dat to &lt;path&gt; - &lt;kind of error&gt;


This is the most common type of error:

error copying wallet.dat to &lt;dest&gt; - boost::filesystem::copy_file: Access is denied: "&lt;src&gt;", "wallet.backup"


This means Bitcoin is trying to backup your wallet to C:\Program Files\Bitcoin. You don't have the permissions to backup there. To fix this, run 

backupwallet "C:\Users\&lt;username&gt;\AppData\Roaming\Bitcoin\wallet.backup"


instead.