For your info only:
Just trying something to get the installation running, but I only get the selection choice of DB(PDO). Trying to install gives (of course) the enclosed example error message.
I tried to change coding on different relevant places to MySQLi but no succes up to now.
Using PHP 7.2.1
MySQL 5.6.37
Apache server 2.4.29
User agent: Chrome/64 (Windows 10)
So if we do not support mysqli anymore, then we also have to migrate the installations of J3 which are on mysqli to mysql during the upgrade to 4?