Not to beat a dead horse, which I must, but I mentioned ALL of the courts that other's have repeated.
If OP really wants to play 4.0 level tennis in SF, he'd/she'd need to get over to the courts, meet some players, and figure out the best hitting times.
I grew up mostly at GGPark. It's still pickup on about 1/4 of the courts, while the other's are preplanned action. You go there, you look like a capable tennis player, you be nice, you take your chances, just like any other courts.
I'm sure some of you's and me are testing the waters here. I'm a mobility challenged 4.0, usually play much better in doubles.