Emacs is build from the Launchpad sources, which is based on the Debian emacs-snapshot (which is not maintained any more).
The cassou PPA only has 24.3 builds available.
Do you know any other ways to install it on Ubuntu? Maybe some PPAs or deb packages (now I'm trying to build it from source)?
First off, you should download the Emacs 24.4 source tarball:
cd ~
mkdir emacs-src && cd emacs-src
wget http://mirror.team-cymru.org/gnu/emacs/emacs-24.4.tar.gz
tar xvf emacs-24.4.tar.gz
Now, you want to compile the Emacs binary.
sudo apt-get install build-essential
sudo apt-get build-dep emacs24
cd emacs-24.4
./configure
make
sudo make install
If the system complains about any dependencies, just manually install them.
Sources: (Ubuntu Handbook) and @namingFailed's comment.
If you want to be nice to the community, you could always maintain your own PPA with Emacs 24.4 in it. That's another question, however. I might bring one of my own up later...
