Ubuntu – Vga error after automatic update in 16.04

apt-get does not use the global proxy settings. Try adding the proxy settings to /etc/apt/apt.conf:

Acquire::http::proxy "http://<your-proxy-url>";

Or just run the following commands in terminal:

sudo su
echo 'Acquire::http::proxy "http://<your-proxy-url>";' >> /etc/apt/apt.conf

Ubuntu 16.04 will be shipped with kernel 4.4.

If you upgrade from 14.04 to 16.04, you will get kernel 4.4 installed.

Ubuntu kernel numbers do not match exactly the mainline versions. It will be 4.4.0-xx, that will be based on the latest 4.4.x mainline version.

You can install kernel 4.4 to Ubuntu 14.04 without upgrading to 16.04 by running in terminal

sudo apt-get install linux-generic-lts-xenial

Regarding "how stable is 4.4.6", it can't be answered as it will be based on opinions.