Install Oracle VirtualBox-4.0 in Natty

Add one of the following lines according to your distribution to your /etc/apt/sources.list:

http://download.virtualbox.org/virtualbox/debian natty contrib deb
http://download.virtualbox.org/virtualbox/debian maverick contrib non-free deb 

http://download.virtualbox.org/virtualbox/debian lucid contrib non-free deb 

http://download.virtualbox.org/virtualbox/debian karmic contrib non-free deb 

http://download.virtualbox.org/virtualbox/debian hardy contrib non-free deb 

http://download.virtualbox.org/virtualbox/debian squeeze contrib non-free deb 

http://download.virtualbox.org/virtualbox/debian lenny contrib non-free deb

(Up to version 3.2 the packages were located in the non-free section, starting with version 4.0 are located in the contrib section)

Add the Oracle public key for apt-secure and register

wget -q http://download.virtualbox.org/virtualbox/debian/oracle_vbox.asc -O- | sudo apt-key add -

The key fingerprint is

7B0F AB3A 13B9 0743 5925 D9C9 5442 2A4B 98AB 5139 Oracle Corporation (VirtualBox archive signing key) 

To install VirtualBox, do

sudo apt-get update
sudo apt-get install virtualbox-4.0

Ubuntu/Debian users might want to install the dkms package to ensure that the VirtualBox host kernel modules (vboxdrv, vboxnetflt and vboxnetadp) are properly updated if the linux kernel version changes during the next apt-get upgrade.

sudo apt-get install dkms

source: http://www.virtualbox.org/wiki/Linux_Downloads