try adding <script type="text/javascript" src="log4javascript.js"></script>
in the code seems it is not able to get the reference of the main script.
see if this can help you
<script type="text/javascript">
// Create the logger
var log = log4javascript.getLogger();
// Create a PopUpAppender with default options
var popUpAppender = new log4javascript.PopUpAppender();
// Change the desired configuration options
popUpAppender.setFocusPopUp(true);
popUpAppender.setNewestMessageAtTop(true);
// Add the appender to the logger
log.addAppender(popUpAppender);
// Test the logger
log.debug("Hello world!");
</script>
enter code here
use this link