With the new release strategy in mind, what is the current status of Joomla 4.x? Has the development already started? Is there something on Github? Is there a google group especially for 4.x?
The development strategy for the Joomla CMS is currently being revised. There is going to be at least a 3.3 and a 3.4 and probably more incremental minor versions at irregular intervals. This is different from the previous pattern where we'd go 3.5 and then 4.0 six months apart. It's not known when 4.0 will be released but my guess is it's not going to be necessary to think about until at least the end of 2015.
Development hasn't started on 4.x at all and there is no Google Group specifically for that version.
As far as I know there is no official information about Joomla 4.x. But there is Joomla CMS Distributions project which potentially could be the base of Joomla 4.x.
I think there's some idea to develop Joomla Framework first, and re-write everything on Joomla Framework to build a new CMS.
http://issues.joomla.org/ is already based on Joomla Framework.