I feel that bandwidth should be a major focus of PS4. It wasn't really with PS3. The PS3 was, at least in one major way, a step back from PS2. The RSX GPU lacked any kind of high-bandwidth EDRAM. RSX only had an external memory bus (128-bit) to the GDDR3 memory--Half the bus width of the G70/7800 for PCs which were 256-bit. The PS2's Graphics Synthesizer had a small amount (4MB) of incredibly high bandwidth (for the time) EDRAM @ 48 GB/sec thanks to its on-chip 2560-bit bus. Now I recall a statement by Nvidia's Tony Tamasi from 2004 where he said that top-end games may need upto 3 TeraBytes/sec of bandwidth.
"The bandwidth requirements of game platforms and graphical applications have been growing exponentially," Steven Woo, Rambus' senior principal engineer at Rambus, told Tom's Hardware Guide. "About every five or six years, it goes up by a factor of 10. PlayStation 3, for example, will have a memory bandwidth capability of 50 GByte per second." If this trend continues, projected Woo, a theoretical 2010 model "PlayStation 4" could require ten times the memory bandwidth as next year's PlayStation 3. A statistical projection made in 2004 by NVIDIA's Vice President of Technical Marketing, Tony Tamasi - cited by Woo - anticipates that a top-of-the-line 3D game could conceivably require memory bandwidth of 3 TByte per second.
http://www.tomshardware.com/news/xdr2-quintuple-memory-data-transfer-speeds-2007,1152.html
That sort of bandwidth is not going to happen with any kind of main system or graphics memory. Even Rambus is "only" going for 1 TeraByte bandwidth with their Terabyte Bandwidth Initiative. I think the only kind of memory that could reach Tony's projection of 3 TeraBytes/sec would be
embedded memory. The Xbox 360's Xenos GPU had 10MB of EDRAM which provided 256 GigaBytes/sec bandwidth between the EDRAM and the processors on the daughter die. That was 2004 technology released in 2005. I would imagine, all these years later, that 3 TeraBytes/sec (12x Xenos' EDRAM bandwidth) might be possible.
