Ah, should've figured that, looked at the tl;dr version without even trying to put it together in my head.
What "other half"? From the quote in the OP, that's it. 4 * 4Gbit = 2 GB.
What we don't know yet is how fast the eDRAM is.
... either I am confused or the majority of people in this thread are. Wii U does not have split memory pools, except for the eDRAM.
Well, I kind of expect having MORE can, when used appropriately, balance out being a bit slower, but it will almost certainly not be much help against the next Xbox/PS.