I will present how to install nessus version 4.2.2 on your backtrack 4 final. You will have to start by installing the annexes that you need.
apt-get install libqt4-core libqt4-gui libqt4-network libqt4 libqtcore4-script libqt4-xml libqt4-dbus libqt4-test libqt4-svg libqt4 libqtgui4-opengl libqt4-designer libqt4-assistantThen you will have to download the latest version of Nessus. Take the ubuntu version 8.04 (36/64) depending on your architecture. Then register to receive an activation key (put a valid email address.