If you are currently testing the 64-bit client, in order to patch up to 4.3.4 you will need to remove the 64-bit files to allow the patching process to complete.
With the 64-bit client files in place you will receive the error: Blizzard Updater failed with an internal error.
Simply delete the 64-bit files listed below from the World of Warcraft folder, and the patching process will resume after going through a short repair process.
World of Warcraft-64.app
A new version of the 64-bit client files will be available for 4.3.4 after the patch is live. Please keep an eye on the 64-bit client forum thread for updated files. <3 Kalisto