http://askubuntu.com/questions/633919/how-install-postgresql-9-4

This command installs the recent version for your distribution:

sudo apt-get install postgresql

postgresql-9.4 is available since Utopic.

_Source

Here is a step by step guide to install Postgesql 9.4 for your Ubuntu (Utopic,Trusty,Precise,Lucid) version.

sudo add-apt-repository "deb https://apt.postgresql.org/pub/repos/apt YOUR_UBUNTU_VERSION_HERE-pgdg main"

For trusty:

sudo add-apt-repository "deb https://apt.postgresql.org/pub/repos/apt/ trusty-pgdg main"

To get your ubuntu version:

lsb_release -sc

Now import the public key and install:

wget --quiet -O - https://postgresql.org/media/keys/ACCC4CF8.asc | sudo apt-key add - 
sudo apt-get update
sudo apt-get install postgresql-9.4