OS4.0 Pre-release Update 1 was released on 10th October 2004.  Version numbers are:

Kickstart 51.15
Workbench 51.1
U-Boot 1.1.1 (04-Oct-2004)

This release is available as a free update and bundled with new AmigaOnes. It adds support for the MicroA1 (渙1).  The official list of things included in the update is:

* Kernel-Level Altivec support.
* Much improved compatibilty/speed of 68k emulation (JIT not included yet)
* Improved 68k FPU emulation
* Fully PowerPC-native Picasso96/graphics system
* Fully PowerPC-native MUI
* USB support for input devices
* Support for onboard serial and parallel hardware
* Driver support for the SiI0680 IDE adaptor (including UDMA support)
* Improved PIO mode VIA driver
* Improved Internet capabilities and better ethernet support.
* Printing support
* Improved audio hardware support, including new drivers.
* Comprehensive bug fixes, speed improvements, added features and functionality across all system components.
* and much more...

Shown in the Workbench 1 screenshot are some of the following features:

* Underlined icon is a filesystem link (in this case a soft-link to a disk, the information window below shows where it links to) (v50)
* Default icons can now be made transparent (such as the Projects icon, the underlined Music one is a real icon) (v51)
* "Assign" and "Mount" options on the "insert disk" requester. (new for v51)
* The little boxes on the Shell and USBInspector icons signify that they are "left out" (new for v51)
* Screen titlebar is configurable (v50, updated in v51)
* DOS Preferences (new for v51)
* Workbench Preferences (updated for v51)
* USB Preferences (new for v51)
* USB Inspector (new for v51)
* KeyShow now correctly shows the layout of a PC keyboard (v50 showed an Amiga keyboard - I assume it now looks at the keymap setting in Input prefs)
* The Grim Reaper (updated for v51)
* Ram Disk is now RAM Disk (v51)