"...the resolution afforded by say a 4x5 format would make the diffraction from a small aperture like f45 basically negligible. "

Resolution has nothing to do with format size. F:45 on 4x5, using a 150mm lens is noticeably softer than f:22 with the same lens. The ratio of enlargement is usually much smaller with 4x5 though, (as well as shallower depth of field due to longer lenses) so smaller apertures are more commonly used on large format.