VirtualBox Manager adds some of the features of the virtual machine lists in Parallels Desktop and VMWare Fusion. It now provides preview images of the current state of virtual machines. Users can sort the list and edit virtual machine settings from this window. VirtualBox 4 also makes it easier to create shortcuts to launch virtual machines from the Desktop, without having to open VirtualBox Manager.
Version 4 also enables users to easily delete a virtual machine and all related files. Each virtual machine now has its VM files and setting files stored in one folder, which, in Mac OS X, is located in the user's home folder. Installing guest OS's may be a little easier with support for Apple DMG disk images as the virtual CD or DVD. (Previously, ISO disk images were supported.) Verison 4 can also run virtual machines created with other software using the OVF and OVA vm formats.
The new version updates now offloads virtual networking to the host computer's hardware, increasing network throughput. New port-forwarding rules enable server virtual machines, including Mac OS X Server (on Macs only), to run using NAT networking on a user's machine. The host OS can then connect to the server running in a virtual machine. (Parallels and VMware also support this.) Access to to network storage is faster on 1 Gbit Ethernet due to a new asynchronous I/O model for networked (iSCSI).
VirtualBox 4.0 can now run larger virtual machines, such as Oracle datacenter-class products, on smaller 32-bit desktop machines. The new version supports more than 1.5 or 2GB RAM on 32-bit hosts.
Download and installation is now a two-step process. You download a base package for your host (Mac, Windows, etc), then download an extension pack that adds support for USB 2.0 devices, among other things.
If you've tried VirtualBox 4.0