Haven't been through a full SF winter but I can't imagine a 5/4 is ever needed. Water temps don't dip below 50 and 4/3, booties, gloves and hood should be toasty enough. Been surfing the summer with various combinations of 4/3, 3/2, (no) booties depending on air and water temps. People wear strange combinations of rubber out here (e.g. 4/3, no booties + hood), so have the full array of gear and figure out your favorite outfit(s).
don't listen to that kook with the 3 mil. You're going to need a 4/3 suit, booties, hood and gloves. If you lived near the beach and surfed couple days a week, you might be able to get away with just the 4/3 and booties, but you'll never be acclimated enough driving all the way from manteca.
If you shave your head or are bald, you might want to wear the hood all year...funny looking at first, but a lot of people do it because it's warmer, keeps the sunburn to a minimum and it's the easiest way to avoid surfer's ear.