Hi Nadja,
Since 2018 we are talking about our plans for Joomla 4
Anyway, to answer your questions:
1. Yes, the particles should be ready for Joomla 4. I mean, the particles are CMS independent, they depend on Gantry 5 only. So as soon as Gantry 5 is Joomla 4 compatible you will be able to use our particles as well.
Joomla 4 support will come with Gantry 5.5. For more details, please have a look at
this discussion.
2. Regarding our templates - please have a look at
this blog post. Once Joomla 4 and Gantry 5.5 are officially released, we will need about 2 months to release the Joomla 4 version of our templates.