As expected, PSP Firmware Update v4.05 is now available for download via Network Update or linked below.
Once again, to quote Eric Lempel via [Register or Login to view links]:
"I wanted to give a quick heads up to everyone that a new PSP firmware update, v4.05, will be released soon. We know that more and more of you are listening to music on your PSP, and this update will add new visualizers for music playback, among other features. Enjoy!"