Summer Activities/ Pre-school for My 4 Year Old

here's a link to the San Leandro park/rec section on day camps
http://www.ci.san-leandro.ca.us/rec/ag2009suSummerCamp.pdf

The Oakland YMCA has summer camps

Hayward park/rec
http://www.haywardrec.org/camps/camp_letters.html

Good luck!

There are a pretty decent amount of things that you can take young kids to during the day. Things like reading hour at your public library but for those you would have to stay with him. If you're looking for a day-care sort of thing I would suggest joining Berkeley Parents Network and asking if anyone has a nanny they're looking to share. That way your child can be with another kid their age and the nanny can take them to the park and other fun activities.
Good luck!

You might try the city's activity guide. I live out in Pleasanton and the have a lot of activities. Again it may be harder to find things to drop her off at, since she is so young. I know the preschool has Summer Camps, something like that may work if there are still openings. Even if you don't live in that city, you can still attend, the fee is just slightly higher. Here is the Pleasanton guide to look through - http://www.ci.pleasanton.ca.us/pdf/AG-summer09.pdf A lot of city's have, Dublin, San Ramon, Livermore etc, so check out a few cities around you and see if that may work for you.

Good Luck!

Look into the Exploritoreum in SF, in the Palace of Fine Arts, by the Golden Gate Bridge. The first Wednesday of every month is free admission. Pack a picnic lunch to save money.
http://www.exploratorium.edu/visit/hours_admission.html

Also, if you can invest in an Oakland Zoo membership, you can get a Family zoo pass for $85 that includes free parking, entrance for up to 2 adults and 4 children over 2. You can go whenever you want and there are tons of things to do there. I go with my boys and pack a picnic lunch (because the food there is pricey and not worth the cost).
http://www.oaklandzoo.org/support-the-zoo/membership/beco...

Best wishes to you!

Caterpillar Kids Club is a fun program that you and your son can do together this summer. The class meets at a different outdoor location on the peninsula once a week such as farms, preserves, redwoods, beaches including music, art, hiking and exploration. The class is 1 1/2 hours and it offers lots of opportunities for learning and socialization. Www.caterpillarkidsclub.com.