Only two cores are used at any one time. The chip has two fast cores (which use a lot of power) and two slow cores (which use much less power). When the phone doesn't have much to do, it uses the slow cores to save battery, and when it's heavily loaded, the fast cores come into play. It's known as the ARM "big.LITTLE" architecture.
I will leave you to decide whether this configuration really counts as "quad core". This approach has been used by other manufacturers for some time, although it's not caught on as quickly as I expected.