Based on: http://askubuntu.com/questions/645600/how-to-install-octave-4-0-0-in-ubuntu-14-04

sudo apt-add-repository ppa:octave/stable
sudo apt-get update
sudo apt-get install octave

This obliterated the existing PTB directories, so reinstalled according to:

http://psychtoolbox.org/linux/

sudo apt-get install octave-psychtoolbox-3