Well, it depends on how your downloader settings were configured in the past two weeks.
It’s been a long night with Blizzard bringing the Live Realms, PTR and even the Beta servers offline at around 12am PDT for maintenance. Many folks are really gonna smack their forehead on their keyboards for forgetting to change the background downloader settings because this patch is really big.
Those of you who kept the Launcher settings to download patch files while playing and after exiting WoW are in the good side of the force. The patch had Stage 1, 2 and 3. It has been downloaded for a few weeks on my end somewhere around 4.73 GB size. Up until 3am EST, I kept launching the Launcher.exe and it would simply check for file integrity. Meaning there was no patch yet. Passed out, and woke up to find out all I needed to download was a mere 100MB file because I already had 4GB downloaded while playing the game the past weeks.
Bashiok also mentioned this 100MB file to download if you already had downloaded the core files for Stage 1, 2 and 3. However, don’t hold your breath. You won’t be able to play the game just yet.
After that 100MB was downloaded, and the entire patch 4.0.0 was installed, the new Launcher.exe started downloading patch 4.0.1 which has a size of 1GB.
Whoa? Don’t be alarmed. With this new launcher, you can play the game when the bar reaches yellow. Only problem is that you might find undesirable gameplay quirks here and there, but you might be able to do your dailies, auction activity, check the new UI, reforging, etc.
If you open your World of Warcraft\Updates folder and have WoW-126.96.36.19958-188.8.131.5211-NA-Stage-# and have 1, 2, and 3, then you’re all set. The final part (around 100MB) isn’t available yet, but when it is it will finalize the patch download and begin the install of 4.0.1.
If you don’t have those files in your Updates folder, go to your main World of Warcraft folder and manually run the BackgroundDownloader.exe
If you have stage 1, 2, 3, then you get the final 100MB and you’re set to login with all data intact (playable).