I know someone who's was doing 5 subjects last year (our second year). It's a lot of work. If you need to take up a part time job (she didn't) I wouldn't recommend overloading.
4 Courses + over 10 hours a week at a job is bad enough.
An awesome course, for anyone interested in Science or Philosophy is Math1007 - The
Poetry of the Universe (also done as a second year PHIL course).
The Lecturer (Dayal Wickramasinghe) is awesome, only 2 lectures + 1 tute a week and minimal assessment throughout the year.
A very interesting and laid back course for anyone interested in stuff like (from ANU site):
the overall structure of the observable universe; space and time; relativity; black holes; phenomena and reality; anthropic principles; and extraterrestrial life. Each topic will be introduced in a historical context emphasising how it has evolved over time and is still evolving.