4 weeks is plenty enough to explore those parks. What I was disappointed about though in that region is the towns. Darwin is cute, but the rest is awful. Exmouth has nothing for it, there's a big ex-American military base on the top of the peninsula, and Cape Range NP with its canyons has awful roads and layout. Karatha/Dampier is a dump, there's no other word for it, only Point Samson is kinda cute. I haven't seen Broome, but I haven't heard any good thing about it. So my advice is, camp in NPs and other wild places as much as you can. So you're flying from Sydney to Darwin, and then Exmouth back to Sydney ? How does it work ? Renting a camper one way would be insanely expensive, wouldn't it ? For Purnululu you need a sturdy 4x4 and an offroad trailer.
Anywyay, regarding parks, you're forgetting Litchfield and Gregory, don't miss them. Litchfield is lush with great waterfalls, rockpools and termite mounds. Gregory is drier but wild. For best info you should buy a couple of books : the first ones would be the Hema Kimberley Atlas & Guide (
http://www.mapworld.com.au/products/kim ... tlas-guide ), and the Hema Pilbarra Atlas & Guide (
http://www.offroadequipment.com.au/prod ... las-guide/ ). They're A4 size guidebooks with great maps, great info and quality pictures. A must. For "secret" and out of the beaten path stuff, there's
http://www.kimberleyaustralia.com , where you can buy guides here :
http://www.kimberleyaustralia.com/desti ... p-end.html . They're quite expensive, and they're done in MS Word so it doesn't look pro, but the woman who writes them knows all there is to know on the Top End and the Kimberley, and it's full of honest and useful advice, plenty of non-NP destinations that most tourists miss, such as Umbrawarra Gorge Nature Park. As for walks themselves, there's only one good book for the Top End, Take a Walk in Northern Territory (
http://www.takeawalk.com.au/Z8Main.htm ). There's nothing for the Kimberley, and for the Pilbarra I've only found an old book which was pretty useless to me. If you've got time, don't miss Millstream-Chichester, I know it's not very wild because mining pipelines and other stuff got here and ruined some views, but it still feels pretty special. Karijini also has been ruined by the mining nearby, with awful roads and expensive accommodation. But the gorges and rockpools are still lovely. Are you gonna do the Gibbs btw ? If I had any advice it would be to do Darwin to Karratha instead so you can explore the Kimberley in full. This would require some 4WD experience though, but then again so does most of your trip. If you still go to Exmouth, remember that July is the last month the sharks are here.