Where is 16.04.4 LTS?

It was delayed due to the Meltdown and Spectre security issues. This took up a lot of everybody's time while things were patched and tested. There are still patches coming out for various CPUs. It's a complicated issue and the fixes are far from ideal.

Anyway, the new release date was recently announced as the 1st of March 2018.

See https://lists.ubuntu.com/archives/ubuntu-release/2018-February/004287.html

From there:

As announced previously the release of the 16.04.4 point release has been delayed. Seeing that things are now settling in, we have set the 1st of March as the new planned date release date. We expect to have all the required pieces available in the archive by that time and will provide images with all the necessary security fixes in place.

On behalf of the Ubuntu Release Team,

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Łukasz 'sil2100' Zemczak
Foundations Team