Well, it depends. Number of cores and frequency are only part of what give you performance. Not all 3GHz cores perform the same - far from it. But if we assume the same architecture, and linear scaling, for the sake of argument, the answer is not really. The raw processing power would be roughly the same. If the app primarily runs in one thread, like many games do, the 4GHz chip would likely outperform 2x2GHz for that workload, as one of the cores would be maxed out, and one of them mostly idle. If on the other hand, your app can use multiple cores at once efficiently, or you are running several apps at once, then processing can be evenly spread over the cores, and things can happen in parallel. In this case the 2x2 would probably be slightly better than the 1x4. Generally, systems with more cores are a better fit for modern operating systems and apps, and generally will use less power and produce less heat. Gaming still wants great single core performance though, and there are some other workloads that are primarily single thread. But the trend to many cores is clear - iPhone SE has 6 cores. Macbook Air has 8 cores.
