If you are currently playing games from your memory stick and if you are running on custom firmware, DO NOT, i repeat, DO NOT install this update. Installing this firmware update will prevent you from playing games from your Memory Sticks and will disable your PSP’s Homebrew Capabilities. So if you want to continue playing games on your Mem Sticks, DO NOT install this update. And how many times have i said DO NOT install this update?
You can view your system settings if you want to know if you are running on custom firmware by going to:
SYSTEM SETTINGS -> SYSTEM INFORMATION
If you have an m33 beside the System Software version that means the you are running on Customfirmware, and i strongly advise you guys to avoid this update and just wait for Dark_Alex to release an updated version of the custom firmware.
This 4.05 update only adds a new visualizer effect when playing mp3′s on your PSP, which is a lame update to say the least…
But if you still want to update, you can get the 4.05 update from sony here.