Install Octave 4.0 on Mint 17.1/Ubuntu 14.04

Simple method

In a terminal, write

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

That's it!

Hard alternative

Octave version 4.0 is out, and the good news is that it has a classdef feature similar to Matlab. I will soon use it to update my FVTool so that it can be used with all functionalities in Octave as well.
The problem is that it takes a while before the new version of Octave comes to the software center of Ubuntu. I decided to compile and build it, although I've never had a good experience with building tools from source. This time however it worked however without any issue on my laptop and my pc. I used the procedure I found here. I had to install some more libraries on my Mint 17.1 operating system.
First, install these dependencies:

sudo apt-get install gawk gfortran gperf flex libbison-dev libqhull-dev libglpk-dev libcurl4-gnutls-dev libfltk1.3-dev librsvg2-dev libqrupdate-dev libgl2ps-dev libosmesa6-dev libarpack2-dev libqscintilla2-dev libreadline-dev bison icontool llvm-dev libfftw3-dev libhdf5-serial-dev openjdk-7-jdk transfig  libgraphicsmagick++1-dev libsndfile1-dev

Then, download the octave source code for version 4.0.0 from here, and extract it. Go to the new octave-4.0.0 forlder, and in a terminal window type

./configure --enable-jit
make
sudo make install

It takes a while, so go and pour a nice cup of coffee for yourself. If you get any error after running ./configure, find the missing dependencies here.
If the installation is successful, run octave by typing octave. It should launch the user interface of octave which is enabled by default in this version.
One other good news is that now you can run octave in a Jupyter notebook, by installing octave kernel and oct2py:

sudo pip install oct2py
sudo pip install octave_kernel

It is still not possible to use octave kernel. Go to /usr/local/bin and type

ls -la

You will see that octave is linked to the octave-4.0.0. You need to unlink octave and create a symbolic link to the octave command line interface or octave-cli. In the terminal window, type

sudo unlink octave
sudo ln -s /usr/local/bin/octave-cli octave

Now your octave kernel should start without any problem. Open a Jupyter notebook, and change the kernel to octave.