Many people won't sell you a knife because you're not 18, but if you're anything like I was when I was that young, you'll be able to get it without too many problems, but understand what the laws are, especially in California. Also, if you're going to use a 4-inch locker for defense, take some training! If some guys are hassling you and you escalate the situation by verbally referring to the weapon (which it has now just become, whether it's on you or not); physically producing the weapon in a display of resistance (legally, brandishing), or physically using it in a physical confrontation (assault), you will be in a world of hurt. At your age, frankly, you would have to meet an almost impossible criterion.
But to answer your question, many on this board like the Buck 110. The Cold Steel LARGE Voyager and the Recon 1 are also very good knives. Finally, the CRKT M21-04 is one of me absolute favorites.