I would think about another park you might want to go to.
Universal Studios isn't for younger kids in my opinion. They have water park areas, virtual rides, fair games, shows and the studio tour. Out of that list the virtual rides sometimes have age or height requirements, the fair games are a lot of money, most of the shows wouldn't be appropriate for a 4 year old, and the studio tour is ok, but some get bored on it (the biggest part of that now is the King Kong 3D experience...the rest of it isn't as exciting).
For the ages of your kids I would consider Disneyland, Ca Adventures, Legoland, SD Zoo, or maybe Knotts Berry Farm (they have some rides for younger kids).
If you don't want to go all the way to San Diego, but don't want to do the whole Disneyland thing...check into Knotts, it's close to Anaheim.
I'm totally not trying to cursh your plans....I live in So Cal and I am a season pass holder to a lot of the theme parks.
Everytime we go to Universal we leave our 2 year old at my inlaws for the day and go. It never fails that people bring small kids and they take them into something and the kids freak out (like the terminator show with loud gunfire and smoke) or the House of Horrors (scary walk through haunted house thing), even the Simpsons ride had kids screaming they wanted off. Even the studio tour had kids crying during the King Kong 3-d part.
It's just from exerience. I hope this helps in some way.
Have fun on your vacation!