Hi James,
For what you describe you dont need 4wd. 4wd is more impirtant you go to the north. Around coral Bay and more north there may be more dirtroads where 4wd is recommended but how often will you drive the 1k km up? If you do you could rent for a short amount of time. For perth around you definitely dont need it.
Actually on my trip the only place where i missed 4wd was the entry to wedge Island. There is no go without 4wd.
Enjoy your stay ;)