Samsung is forced to use Exynos in all the S4 phones or they would be making a huge mistake.
The Exynos is not just another chip, it's a significant change in chip technology it's a whole new concept in processing architecture and the first to use ARM's big.LITTLE technology. Samsung is claiming lots of benefits of their chip like over 70% savings in power consumption. If Samsung was to put Exynos in one version of the S4 and the Snapdragon in another version and market them as equivalent phones then they'd loose the differentiation of the big.LITTLE technology and make it appear like it's nothing special out of the gate and negate all the perceived benefits. Future Exynos devices would carry less clout and at the same time Samsung would be admitting that Snapdragon's chip technology is as good as the new Exynos technology.
The Exynos is not just another chip, it's a significant change in chip technology it's a whole new concept in processing architecture and the first to use ARM's big.LITTLE technology. Samsung is claiming lots of benefits of their chip like over 70% savings in power consumption. If Samsung was to put Exynos in one version of the S4 and the Snapdragon in another version and market them as equivalent phones then they'd loose the differentiation of the big.LITTLE technology and make it appear like it's nothing special out of the gate and negate all the perceived benefits. Future Exynos devices would carry less clout and at the same time Samsung would be admitting that Snapdragon's chip technology is as good as the new Exynos technology.