Upgrade problems from syslog-ng 3 to 4
Version 4 of syslog-ng works perfectly well in version 3 compatibility mode. However, if you want to use the syslog-ng 4 features, you need to be aware of some significant changes. If you have a simple configuration, like those in Linux distributions, then simply rewriting the version string is most likely enough. However, if you use PatternDB or JSON parsing, any Python code, or an Elasticsearch, or MongoDB destination, you have to be aware of the changes.
From this blog you can learn about type support, how this can affect you, changes in Python support, and some tips how to prepare for the upgrade: https://www.syslog-ng.com/community/b/blog/posts/upgrade-problems-from-syslog-ng-3-to-4
