Can you get a sheet of lightweight plastic (solid, not mesh), and cover the garden while you are spraying?  With a 4'x4' garden, I think that should not be unreasonable.  Maybe keep the garden covered for a couple of hours after spraying until the herbicides dry on the plant.

I don't know how effective washing plants off would be.  If herbicides get on the plants, and the homeowner washes it into the soil, that may not be much help.