What is the timeframe for supporting Joomla 4.0? Is a compatible Civi version in the works for it, although Joomla 4.0 is still in alpha?
Currently, 2019/08/20, there is no activity with regards to Joomla 4.0 compatibility with CiviCRM - I would imagine that this will be prioritised when Joomla 4.0 enters the Beta phase.
It would be useful for the CiviCRM community if those interested in this would test Joomla 4.0 and CiviCRM Stable and report any issues to the CiviJoomla issue queue.
Since the initial revision of this answer (and evidenced at the issue queue link above) there are now patches/issues for CiviCRM on J!4 and there’s definite progress/momentum towards getting a better J!4 + Civi than J!3 + Civi.