This website thinks that by creating 4 different possible recognisable states, rather than two that Intel has effectively created double the storage space in the same area. However, the way that binary works, it means that there is actually 16 times the ammount of storage space in the same area.
Each character stored requires 4 bits (or 1 byte) to identify it. The same information could be stored in 2bits. Leading to the idea that you could have twice the storage room.
However, if you consider that they will still work in bytes, the equation changes. A byte with two possible states has 16 different possitions (2x2x2x2). However a byte with 4 different possible positions has 256 possitions (4x4x4x4). This is 16 times the storage space per byte.
QED