Hello
I also had the problem that after installing the JA Purity iV template, a white page appeared, both in the frontend and the backend, when I tried to open the template for editing. After activating the Joomla 6 backward compatibility plugin, everything worked normally.
It's actually strange that a complete fresh installation of Joomla 6.0.3 with a Joomla 6-compatible template, JA Purity IV, and the T4 plugin still requires the J6 backward compatibility plugin and doesn't work without it.