You need to either specify where the application should find its log4j.properties, or put it onto the classpath of the application. Where the classpath is varies, but in general WEB-INF/classes should work. Other options depend upon how you're deploying the application.
A better long term strategy is to configure your system so that you can change the log4j.properties depending upon the environment. When you're in production, you won't want all of the debug information to appear. Look at the answer to this question or this question for more ideas. One strategy is to define a variable on the command line which gets picked up and defines a directory which contains your configuration files. This works for Tomcat, but there may be other, better, strategies for Weblogic.
It is not a good idea to change the configuration of your server, in particular, don't replace the log4j.jar or log4j.properties in your server directories. The server will depend upon the version that it was designed to use, which may or may not be the same as your version. You can do everything you need to do by changing the war that you're deploying.