http://techon.nikkeibp.co.jp/english/NE ... 11/126348/
At the 2007 International CES, Hitachi, Ltd. has demonstrated playback of a Blu-ray Disc (BD) with four recording layers at its booth. Featuring 25 GB capacity per layer, a disc can record up to 100 GB data.
There have been other academic reports of creating four- or six-layer disc media and demonstration of picking up signals from those discs using test players embedded with a special optical head, but, according to Hitachi, "This is the first demonstration of quadruple disc readout using a player based on an optical head generally used in current BD drives on the market."
An optical disc drive based on the "GBW-H10N" supporting 4x speed BD recording, which Hitachi-LG Data Storage, Inc. announced in July 2006, was used for the demonstration. The four-layer disc used in the demonstration was an outside company's product, but a Hitachi spokesperson said, "We cannot tell the name of its supplier."
You gotta wonder how "minor" the changes were. Still, its a good sign that 4 layer Blu-ray Discs are possible, at least from the hardware end. Producing the media is still a problem I guess.