There have been confirmed reports that the Galaxy S4 will come in two virsions - I9500 powered by Exynos 5 Octa chipset and I9505 powered bySnapdragon 600. I9505 supports LTE connectivity while I9500 doesn't. The octa core models were to be sold in countries that have limited LTE availability.
In latest news, there are reports that 70% of the first batch of Samsung Galaxy S4s will come with Qualcomm Snapdragon 600 CPU, rather then the Exynos 5 Octa CPU. Reportedly, the Exynos processors are having performance issues and there isn't enough time for the chips to ship out before the launch, as demand for the smartphone has been high. Therefore the Snapdragon 600 switch has been termed necessary. The South Korean based company has said the Exynos powered models have been scheduled for mass production in Q2 2013(June- July)
A lot of people will be disappointed by this, as Octa core for a smartphone is something that's completely new and it would easily be the trump card of the Galaxy S4. Non-the-less the snapdragon 600-powered Galaxy S4, still is said to be the fastest phone currently available.
This decision probably works best for Samsung, as performances issues could mean a disastrous outcome for the company. Besides the competition this year has been pretty tight. Samsung cannot afford to release anything less then the best, if they have to stay ahead of the competition.