Just a few days after the update to version 4.0, Sony today made available today version 4.01 of the PlayStation Portable's firmware.
As suggested by the tiny numerical increment, this update is just a bug fix. According to Sony, the update fixes video playback problems and some errors in displaying different languages when conducting searches. The search feature was new for version 4.0.
You can make the update via the usual channels. If you don't know what those are, check out [a href=http://www.jp.playstation.com/psp/update/ud_01.html]the update page[/a] Sony has set up at the official PlayStation.com site.