Hey Mehran,
I disagree with a few of the comments here, and I'm going to tell you my opinion about investing here in Los Angeles.
First of all if you plan to buy a small income property (like a fourplex) it would only make sense to buy something in an area close to you and that you're familiar with. Outsourcing management for such a property isn't cost effective and buying one in area you're not familiar with is risky.
The Valley is a great place to buy now. together with Long Beach it's the only place you can still find good deals in Los Angeles. Remember that California and Los Angeles are one of the fastest growing (population) areas in the country. For rentals this is one of the major facts you have to look at. When values are going to go up again your property is going to be worth a lot more and since people are going to have a harder time finding a place to live they're going to have to pay a lot more rent.
Regarding rent control. This is not always a bad thing. Might even be a great opportunity since the current value is under-market. I know many investors who took this opportunity and payed tenants to move out. They made a whole lot selling the property later when the income was at market value.
If you can't find a good deal on an income property you can also look at condos in the Vally - you can easily get a 9% return on condos. And when the market will go up the same will apply to those condos.
Good Luck!