I think the SLI comment is the part where it became clear that Sony's baiting insiders. Even some of the leaks from different insiders line up with SLI, like requiring a patch to use the extra power which fits with SLI. Obviously Sony would have to be completely insane to do that, so one has to question the rumor sources the insiders are working with after the initial round of leaks.
SLI = nVidia
Crossfire = AMD
Multi-GPU Configuration = Either
On behalf of all PC Gamers I implore you to refer to it either as "Multi-GPU Configuration" or Crossfire. Since the PS4k will likely be using an AMD APU with an AMD-GPU built in, adding an additional AMD-GPU to the board would be similar to AMD's Hybrid CROSSFIRE.
Hybrid Crossfire allowed an APU to Crossfire with a discrete Graphics card. So a PS4k using a more advanced version of this would be within the realm of possibility.
So let's play Theoretic here.
Let's say the PS4k is going to be using an APU based on AMD's upcoming Zen and Polaris CPUs and GPUs respectively.
Now lets say the new PS4k APU is based on the Quad-Core/8-Thread Zen CPU running at 2.4Ghz, with an R9-380/GTX960 class GPU based on Polaris. Now, while a single GPU of that class may not be able to run 4k, but in Hybrid-Crossfire with a GPU similar to the one in the APU tied with 16GB of GDDR5 memory and the general console fine-tuning, would see 4k/30fps actually be realistic.
The other route would be a dual-APU set up. Two Zen Quad-Core, each with an R9-380 class GPU.
I mean that's a hell of a chip to keep under-wraps, but seriously 4-5 years ago, the PS4's APU was pretty decent by today's standards. A 1.6Ghz 8-Core Athlon Processor with an AMD Radeon 7850/R7-265/R7-370 class GPU. Hell, today that would still be a pretty potent APU to build a media PC out of.
So, release about 4 years after the original PS4 helps make those theoretical specs at least plausible.
Also, I use the R9-380 as a performance stand-point. The 14nm Polaris Chip of it's performance class is likely what AMD demonstrated a few months ago against a GTX950 in Battlefront.