The list of repository sources can be found at https://www.virtualbox.org/wiki/Linux_Downloads. For 14.04 ("Trusty Tahr"), it is "deb http://download.virtualbox.org/virtualbox/debian trusty contrib".
Run the command to add apt repository to your sources list:
$ sudo add-apt-repository "deb http://download.virtualbox.org/virtualbox/debian trusty contrib"
Before update apt source list, you have to add Oracle public key. Download the Oracle public key for apt-secure (oracle_vbox.asc) from the same page. It should be like this.
Then run the command:
$ sudo apt-key add oracle_vbox.asc
Update apt-get and install VirtualBox 4.3:
$ sudo apt-get update
$ sudo apt-get install virtualbox-4.3
