Although you could use a Air diaphragm or 12V pump to wash a house it would be slower. Also the lack of pressure wouldn’t be great at knocking any nest or stubborn stains down. The only time i use my aodd for a house is on stucco and limestone, and even than i has to be a lot of area. I would rather not pull that hose and just use a pump up sprayer most of the time.
As for the 5.5 over the 4 gpm. There is a pretty big difference. I have both on my trailer and will always choose the 5.5. With the right nozzles the 5.5 will shoot further, it rinses way quicker. If you use the same xjet you currently are using, if its an unmodified one, then 5.5 will mist more. The 4gpm is really only effective at running a 16in surface cleaner but the 5.5 can run a 20in so less pass on concrete. As for the lower pressure on the 5.5, the only time ive needed more than 2500 psi was when i was running 400 ft of hose to a surface cleaner, so i used the 4/4k. I do a lot of community drives where there are 75+ driveways and the 5.5 is a better machine. Concrete doesn’t need that much pressure to be cleaned. Ive also tested my flow on both machines, i dont know how some are getting more than 4gpm open flow. And cleaning units mean nothing, its a sales gimmick to sell to homeowners.
All that being said if its just you washing than it would be better to hold out for an 8gpm.