If you do not mind installing the Ubuntu version of VirtualBox 3.2 instead of the latest 4.0, install
virtualbox-ose (VirtualBox Open Source Edition) instead of virtualbox-4.0.
virtualbox-4.0 isn't in the default repositories, you've possibly added a 3rd party repository which isn't compatible with the standard ones. If you cannot wait and really want the 4.0 version of VirtualBox, follow the instructions at http://www.webupd8.org/2010/12/install-virtualbox-40-stable-in-ubuntu.html.
To install the virtualbox-ose from Ubuntu, you need to remove virtualbox.org from your repositories. Run the next command to remove all lines containing "virtualbox":
sudo sed '/virtualbox/d' -i /etc/apt/sources.list
Alternatively, remove the three "virtualbox" lines from your /etc/sources.list manually using a text-editor like gedit:
gksu gedit /etc/apt/sources.list
Related launchpad question: virtualbox-ose 4.0 needs packaging
virtualbox-ose in ubuntu 11.04 Natty and newer comes with version VirtualBox 4, no additional repositories are necessary.