Born and raised there.
You won't deal with 4x4's if you shoot the north end of the dunes or head South of Oso Flaco from Main Street. My favorite time is in the winter. much less foot traffic and a much higher concentration of sunny days. People on the central coast of california wait for winter so they can warm up. Even in the winter it's tough to find an untrampled dunescape on the north end of the dunes, but you will not find car tracks.