WOuld have to re-iterate Kramer's advice re the time. Four weeks would get me to 80 mile Beach (roughly halfway between Port Headland and Perth) and that's about it.
SOunds like you want to stick to the coast ... in which case (time depending) I'd go to:
Kalbarri (maybe first stop)
Canarvon (short stay)
Exmouth/ Coral Bay (you need at least 4 days here just to soak it all in)
80 Mile Beach (fish fish fish)
Broome (of course)
And then I'd turn around and come back staying at different places or dashing back to the big smoke 'cause you spent too much time at the above places.
Note that this is a basic trip. I haven't included tours from these areas but a quick chat to visitor info would glean some good info.
I really think 4 weeks is just too short to do the rest of Aus. Just know time to enjoy it all.
Check your PM box too ...