Great coastal vistas, walks, history ( Anzac centre in Albany), wilderness then wine areas.
Places to stay- bush or beach chalet in the the northern part of MR region, chalet around Pemberton, Denmark ( close to Albany but allows exploring of that area) Albany ( near Middleton beach or Emu Point), Porongorup and Stirling Range NP has accommodation nearby too.
3-4 weeks is certainly enough time for a lovely leisurely trip.
If you would like an even longer road trip then head further east- Hopetoun ( for Fitzgerald River NP) - Esperance- Kalgoorlie-Perth. These additional areas give you even more National Parks, Biodiversity hot spots and mining contemporary plus history and an outback feel in Kalgoorlie. 3-4 weeks would give you time to have a huge route and not be rushed.
Best time could be March/April or mid/late September for Wildflowers. Or summer months ( though Kalgoorlie would be hot). Any time actually other than late June- end of August.
Let us know your interests so we can give suggestions.Edited: 27 February 2018, 11:05
Thank you so much, this was very helpful. Good to get some local advise!