You can definitely get 4br rentals for under $400pw around Mandurah and some areas up to Rockingham and also around Bertram (further from beaches - closer to Perth). Mandurah has a big FIFO population, I used to live in Halls Head (Mandurah) and it's mostly nice with good schools, parks etc. There are parts of Mandurah and Rockingham that you would want to avoid. If you so a 'map' search on realestate.com.au you can see where the affordable rentals are.
We've gotten some good value rentals in areas which are coveted by owners but not so much for renters as you need a car to get everywhere. The last place we were in was 4br older (well, WA older = 1970s) on the market for $600k and our rent was $290pw!