Elliotte, your wasp is a female. She looks to be very close to Eric's image of a female Ammophila aberti, from Arizona: (as well as Hartmut's image, that you already linked)
Please note that I think your size estimate is about twice what it should be and these females are rarely over 4cm. In fact, if I'm right about the species ID, then she is most likely closer to 3cm. Maybe they are bigger in California, I'm not sure.
ID reference note:
Here is a bit of the ID info for A. aberti females, that you might like to read: (copied from Google Books)(links are on the info tab)
"... First segment of the abdomen more or less black above, sometimes a black line on the base of the second segment above; the fourth and fifth segments vary from entirely black to red with a black spot above."

This is NOT A. kennedyi, as I have a specimen identified by an expert that bears no resemblance to this...Great image, though, and at some point it may be identified to species.