Debian 8.0 Jessie - Install Mono 4.0

Mono 4.0 was released in May 2015.

In order to install it it on Debian 8.0 Jessie do the following as root or with sudo.

apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 
3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF 

echo "deb http://download.mono-project.com/repo/debian wheezy main" 
| tee /etc/apt/sources.list.d/mono-xamarin.list 

echo "deb http://download.mono-project.com/repo/debian 
wheezy-apache24-compat main"
| tee -a /etc/apt/sources.list.d/mono-xamarin.list 

echo "deb http://download.mono-project.com/repo/debian 
wheezy-libjpeg62-compat main"
| tee -a /etc/apt/sources.list.d/mono-xamarin.list

apt-get update

apt-get install mono-complete

To get development IDE run:
apt-get install monodevelop

I had also to run several times apt-get -f install to force installation of failed dependences and run apt-get install monodevelop to get development IDE.

Reference:

Install Mono on Linux