Hi Lynda,
I did believe Joomla 4 would be here by now, but it seems a few months off yet.
Me too! I did believe that a stable version of Joomla 4 will be released in the beginning/mid of the Summer 2019.
Anyway, back to your question - you will be safe if you start switching to Gantry 5 templates because both we and Rockettheme will make our Gantry 5 templates Joomla 4 compatible.
But in my opinion the safest option is to wait until Joomla 4 is released (should be in 2-3 months, hopefully) and then start updating and switching the templates.
Actually, it doesn't really matter, but still, I would wait for Joomla 4