As we have mentioned in the VirtualBox 4 Beta article, the latest version comes with some fantastic features and improvements. Here is the list of new features available in version 4.0:
# Supports more than 2GB guest RAM on 32-bit host machines
# New display mode with downscaled guest display
# Redesigned user interface with guest window preview
# Supports resizing VDI and VHD images
# Intel HD audio is now available as guest hardware
# Supports copying files into the guest file system by installing guest additions
You can find the full list of features and fixes at the official changelog page. Download is available from the below mentioned link.