better communication between the cores themselves, and the hypertransport bus will be a lot better going to just the one die instead of two.
the one die is necessary for AMD, because they are planning to put level 3 cache into the CPU's, shared by all 4 cores
*edit*
and as for the 4x4 and stuff:
4 x 4 means the ability to put 4 quad cores in one PC.
no, they're not aiming it at gamers and such.
they're aiming it at server PC's. it'll be good for businesses (not the end user, because it is too expensive for even high-end gamers)
it means more cores in one PC, which = less PC's for a certain number of cores
and they're also trying to focus heavily on low power consumption per core
that means less physical CPU's, and less systems, but more actual cores, and a lot better power consumption.
not to mention, with less systems, that means less maintenance, therefore less costs.
so for businesses, 4x4 means better all round on price/performance.