If you need to install specific version of the package in your system, you can list all available package versions with the command:
apt-cache madison varnish
After you know the package version you can install it with:
sudo apt-get install varnish=4.0.2-1
To hold the package on the selected version during the next upgrade:
sudo apt-mark hold varnish
You can also "unhold it" with:
sudo apt-mark unhold varnish