I have a large web project that uses log4j directly, together with many 3rd-party libraries and a mix of logging libraries.
The only benefit I see for switching, is that you can funnel all the logging frameworks through only one framework, which might simplify your configuration.
Probably the main reasons why I moved to slf4j (this only applies to slf4j + logback) is that you can reload the configuration via JMX, which is GREAT when you have a problem that disappears with a server restart.