What to see... what to do in four short days?

I'm a second-generation native of "America's Finest City". If you've never been before, here's what I'd suggest (not necessarily in this order):

San Diego Zoo, Wild Animal Park, Sea World, Balboa Park, Old Town, Tijuana (Mexico), La Jolla, the Cabrillo Monument, a harbor tour, Horton Plaza in downtown (for shoppers), Legoland (for kids).

Beach weather could be "iffy", either warm and sunny or an early touch of "June Gloom".

I also live in America's finest city. The above suggestions are excellent. That is a lot of stuff to do in 4 days. However I would add one more. Visit Coronado Island and tour the famous Hotel Del Coronado. There is an excellent Mexican restaurant with great food, nice ambience and very reasonable. It is called Miguel's Cocina and is about a block from the Hotel Del Coronado across the street. The best way to visit Coronado is to take the Coronado Ferry from the foot of Broadway in downtown San Diego right next to the Broadway pier. It is very cheap and gives you a good view of the city from the water.

By all means, go downtown and walk around the Plaza (cannot remember the name of it), it is along Broadway. Great shops, restaurants, not to mention the award winning architecture.

I am also a resident of this fine city. I am a big fan of the gaslamp quarter (which is home to Horton plaza) I would suggest doing it on a Friday or Saturday night- it really comes alive. Two thumbs way up for Dick's Last Resort.

Also check out Seaport village if you have time. About two blocks from Broadway on harbor, home to a carosel, outdoor ice cream, lots of street performers, amazing views of the bay and Coranoda Bay Bridge and some very cute little shops.

One last suggestion- check with your hotel to be sure they are not hosting a Prom during your stay. Many of the bigger chains are hosting this big event sometime in May and generally the kids stay at the hotel that night. Lots of these turn into all night parties and I have seen the police show up at three in the morning more that once...

Hey! I'm a travel nurse living here
in San Diego and I've entertained
many visitors in 4 short days...so
much to do and see here!
La Jolla,Coronado,and the zoo are a
must!
The zoo could take a whole day if
you want to see it all...get a
kangaroo bus pass or take the bus tour to see the outskirts of the zoo and spend most of your time in the central part of the zoo. Also food is expensive there ($7 for your basic burger and fries) but the best place to eat at the zoo is at the treehouse cafe for the great view!
Go on a weekday to avoid crowds.
Coronado and Balboa Park are very nice on a Sunday. Taking the ferry to Coronado gives you a spectacular view of the island and the city. Though going over the Coronado bridge is an amazing view as well. Don't miss the Hotel Del Coronado though.
In La Jolla don't miss the seals and the sunset, it's incredible!
For your nightlife try Old Town or the Gaslamp district. Plenty of restaurants and bars there.
If you like to shop I highly recommend going into Mexico to Puerta Nuevo (spelling) and Rosarito just outside of Tijuana. I just spent 3 glorious days there shopping relaxing, and eating the best Mexican food ever! Eating lobster in Puerta Nuevo is a must! Easily a day trip.
Email me if you want any further details, I'd be happy to help!
Have a great trip! San Diego is a beautiful city! Such a variety of things to do!

You have gotten excellent advice. However, as a native, May weather along the coast can be overcast with a marine layer so don't expect perfect weather until you drive several miles inland. The recent weather patterns have been in transition so it is hard to know what to expect. This has started out as a very dry year.