The last version of 4WS.Platform is based on Java 11

You can choose between 2 versions: based on Java 7 and Java 11: it is up to you which version to use.
See the "Java 11.txt" file and go on with the installation of Apache Tomcat 9 and 4WS.Platform 6.0.1
You will not lose anything you have already configured, just execute the new installation indicating the db scheme you have used so far.