According to co-CEO of Samsung Electronics J. K. Shin it isn't. He didn't go into details though. Some might say that its only marketing, Samsung doesn't want the users to pay attention only to the S4 variant with eight cores, and all the rest of them to remain unsold. I am telling you that for once the BIG company is telling you the truth(well most of it though).
The Exynos Octa indeed has four cores more than the Snapdragon 600, but it uses only four cores at a time. The chip has four Cortex A7 cores and four Cortex A15. These cores have different features. The A15 is way better in terms of performance, but the A7 is a lot better in terms of battery life. In a scale of 0-100, the A15 uses 100 battery and the A7 uses 30(as seen on the demonstration made from Samsung). The A7 cores are used for processes such as playing videos or lightweight applications to save battery life. On the other hand when the users wants to play games or other resource hungry apps the A15 cores step up. It is a very clever combination or as Samsung calls it big.LITTLE. Having said that this is a new technology and isn't perfect. There are still some issues but lets leave those to the technicians. What matters is what comes to our hands and what we can do with it.
Conclusion:
So a simple way to put it is this: You will use only four cores regardless of the Galaxy S4 variant you have. So the speed that your phone is going to play videos, games, open webpages or anything else will be almost the same.
