I would need much more information if I was to give any sort of price for this.
Why does the house require power washing is it algae, dead ivy or something else that needs removing? What is the surface to be painted ?brick,? or some sort of rough finish? Has it been painted before? Will it require any making good before even starting? What sort of access is available to all walls of the house.Will scaffolding be required? Four bedroomed houses come all sorts of shapes and sizes.The same applies to the wall although iI would presume (always dangerous presuming though) that the wall has easy access to and does not have bushes and shrubs up close Does the wall border a pavement?
Sorry to be so negative but I feel that anyone doing the job would need to see it before any idea of price could be given. I would certainly want to view before giving any indication of cost. Contact a local tradesman is my suggestion.