Use the Log4Xml parser if you are using the Apache Log4 Libraries (Log4Net, Log4J, Log4Php, etc...) with one of the prebuilt XML appenders. The Log4 libraries have more or less standardized in the output XML format which means that the LogViewPlus Log4Xml parser requires no configuration.
You can think of the Log4Xml parser as being exactly the same as the XML Parser, but with a prebuilt configuration that makes it easy to work with Log4 XML files.
Starting with LogViewPlus v2.5.50, XML log file formats can now be automatically detected. This has made the Log4Xml parser less useful as most of the time LogViewPlus should be able to parse this format automatically using the standard XML Parser.