SM is by far the closest. I'm assuming your layover is in SFO and not Oakland or SJ (both of which are international airports). Most of sm orgs are also like in one spot so you might even be able to do a back to back....altho with four hours only I don't recommend. Lol.

Its a long drive from SAN DIEGO to SAN MATEO to just have an hour session.

There's options in La Jolla and Mission Valley, zero in the immediate vicinity of the airport that I know of but those options aren't super far away.  I like SKParadise (www.skparadise.com) personally.

up the I-5 from the airport.  That's your best bet but there is screening you have to pass as there is with most Kgirl orgs in California.  With that said, I live in SoCal and I wouldn't try it with only a four-hour window to get there, take care of business, get back, and get through security, unless it was the middle of the day.  Traffic during afternoon commute times can be brutal.