I see you wrote wrong board but ill answer anyway. with a 4 gpm pressure washer to sufficiently clean you would need a 16 inch surface cleaner… I think the “rule” is 4 inches to every 1 gpm. someone please correct me if I’m wrong.
I currently run a 18 general pump sc on a 4gpm 3500. We have changed the tips to reduce the pressure to about 2000psi. Im still working on the tip arrangement to prevent lines, until then a post treatment works. but we do have a slightly slow walking pace with the 18 that might not occur on the 16