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.
https://www.syslog-ng.com/community/b/blog/posts/upgrade-problems-from-syslog-ng-3-to-4