If i understand it correctly the PS4 uses stacked GDDR5
No, it doesn't, it just has a memory controller on die and memory off die, like a gpu card, its got nothing stacked or on die.
Almost every GPU amd has made for a few years has a ddr3/gddr5 compatible memory controller, its why we've seen the low end cards and sometimes lower midrange with alternative versions with ddr3 for cheaper.
In terms of desktop I would imagine its mostly a case of allowing Kaveri to be sold in different packages, ie one company wants to sell it as a gaming computer in their line up so they pair it with gddr5, higher gaming performance with it not being bandwidth limited, but a small reduction in cpu performance for cpu oriented usage. Another computer will be aimed at every day use, more cpu heavy, workstation style and they pair that with ddr3 for slightly higher cpu performance, lower gaming performance.
On sockets, firstly who cares, secondly you have two options, move forward when new things happen, delay new features and moving the industry going forward for backwards compatibility of products.... there is a reason Windows runs like a dog and mobo's came with worthless(for 99% of people) pci slots, serial ports, IDE connectors for donkeys years after they were dead, because people couldn't move on. I welcome new sockets and new ideas and couldn't give a damn about old socket/mobo support.
Kaveri is the first widely available HSA chip, gddr5 compatible and GCN onboard, I neither expect socket compatibility, want it or think it would be good for anyone.
In terms of stacked memory, stacked would be where you basically have copper traces through the memory so another memory chip above it can directly access the pcb below it, saving space and a lot of power(signals moving cm's on chip and inches off it have hugely different power requirements and speed capabilities). Interposer is where you create what is essentially a small PCB you can plug the chip and memory chips and anything else you want onto it but its on silicon scale with on die chip speeds of communication, interposer should be used before stacking, way before, because its going to be WAY cheaper. What we could see with Kaveri, and gpu's very soon, is an interposer with the chip and some dedicated super high bandwidth memory. One of the key problems with bandwidth off a chip is the pinout, every connection to a memory slot takes up hundreds of pins, going from dual to quad channel takes up space that simple isn't there on a smaller chip, with interposers you have the traces on a minute scale and without any pinout issues, also capable of higher speed and MUCH lower power you can use low power memory with a VERY wide bus(512-1024bit) very easily and get insane bandwidth pretty easily.
Though interposer tech is available(as is stacking) and it was basically expected to appear on some chips this or next year, its more likely its still too expensive and/or yields are too low to see on Kaveri or any gpu's in the next year. But Kaveri with some dedicated on die super wide memory with cheaper slower system memory could make for a very interesting system.
