$ sudo apt-get update
$ sudo apt-get install gfortran-4.9 

gfortran-4.9 is installed in your system.

Make ensure the gfortran-4.9 package were installed using the commands given below,

$ sudo dpkg-query -l | grep gfortran-4.9 *

You will get with gfortran-4.9 package name, version, architecture and description in a table

gfortran-4.9 package basic information: