Where in my Eclipse project should I add the log4j.properties file so that it will work as intended?
This question is already answered here
The classpath never includes specific files. It includes directories and jar files. So, put that file in a directory that is in your classpath.
Log4j properties aren't (normally) used in developing apps (unless you're debugging Eclipse itself!). So what you really want to to build the executable Java app (Application, WAR, EAR or whatever) and include the Log4j properties in the runtime classpath.