It sounds like the setup to a bad joke: How many governors does it take to run California? Except next week, the answer happens to be four. Follow the bouncing ball: Gov. Jerry Brown is leaving tomorrow to lead a trade mission to Mexico. While he's gone, executive powers to to Lt. Gov. Gavin Newsom, until he leaves Tuesday for Special Olympics events on the East Coast. Senate President Pro Tem Darrell Steinberg will then be acting governor until Wednesday, when he goes to Chicago on personal business. That will leave Assembly Speaker Toni Atkins in charge for about nine hours until Brown returns later that day. All are Democrats.