The 0.4.3 update was available on the Spotify website for a period of time, but has now been removed and the 0.3.23 update put back up instead. Instead, you're going to have to wait for the automatic update to roll out to you - which is occurring in batches, presumably with Spotify Premium users at the front of the queue.
If you didn't manage to grab it while it was available, you've got three options. Option 1 is to wait until it rolls out to you. The new update is great, so we don't recommend option 1. Option 2 is to find a friend who downloaded the new installer and get them to send it to you. It should be about 4.0MB.
The last option is to click this link, where we've uploaded the install file to the web. If you choose to do this, remember that you're downloading an .exe file off a third party website. If you're not comfortable with that, then don't do it. At the very least, scan it with some security software before running it.
Oh, and while you're waiting for that thirty-second timer to tick down, go read Pocket-lint's 5 Top Tips for making the most out of the update.
Update: Spotify's put out a statement about the update being pulled.