Another SFO right here. If you had more time, and wanted to get a car and see some touristy things, then I would say LAX. But just to hang out, SFO easy.

LAX if you've got time to leave the airport, SFO otherwise.

Yes.

Nothing about the LAX airport suggests itself as a place that you would want to visit unless you are a big fan of Dante's Inferno as interpreted by the Greyhound Bus Lines. Outside LAX there is the famous In-and-Out and the beach, but you will probably be cutting it fine to even make to to In-and-Out with the time you suggest. Otherwise, you are likely to see the sun in LAX, and that is about it.

SFO is a newer airport, and while it may be cool and foggy, you still really don't have much time to roam around, and the SFO Int'l-to-UA slog is OK, particularly since you can claim your bag, whip through customs, and then give it up again right away. (SFO is a terrible airport for dragging bags around, especially to a rental car on the train through the Adventure of the Parking Garage Elevator System.) SFO has a few decent restaurants inside as well as the museum displays.

I've walked to In 'n Out, had a picnic lunch in the park right next to it, then walked back to LAX all within two hours. It's definitely possible with a 4 hour layover.