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.
The Log4Xml parser will be used by default when LogViewPlus is asked to open an XML file and no other parser configurations are matched. In other words, if you ask LogViewPlus to open an XML file, LogViewPlus will assume it is in the Log4 XML format unless the program is specifically told otherwise.