Whether or not the SMP kernel is installed depends on how the preboot system sets up the system type.
On the second page of the preboot menu, there are four system types. "Older hardware" or "Virtual machine" will install the W4 kernel. Either of the "modern" options should cause the SMP kernel to be selected.
We'd originally thought to make this override-able in the GUI installer, but that would have required much more convoluted logic branching, which seemed like too much for too little benefit. We may revisit this in later versions.