When using the -jar switch to launch an executable jar file, the classpath is obtained from the jar file’s manifest. The -cp switch, if given, is ignored.
Jeff Storey’s answer is going to be the easiest solution. Alternatively, you could add a Class-Path attribute to the jar file’s manifest.
EDIT
Try enabling log4j debugging, and making the path to log4j.xml a fully-qualified URL. For example:
java -Dlog4j.debug -Dlog4j.configuration=file:/path/to/log4j.xml -jar ...