Is DDR4 forward and backward compatible with DDR3?

[stockhausen]

DDR runs at Memory bus clock speed
DDR2 runs at twice Memory bus speed and at a lower voltage than DDR
DDR3 runs at four times Memory bus speed and at a lower voltage than DDR2

It is my understanding that DDR3 is not forward or backward compatible with earlier iterations; so one cannot use now redundant DDR2 memory planes from a borked DDR2 Motherboard (MoBo) in a DDR3 Mobo.

Is DDR4 not forward or backward compatible with DDR3?

I ask this because I want to increase the RAM in a DDR3 MoBo but would like to allow later use in a DDR4 MoBo which I may end up using in the future (or perhaps vice versa - reluctantly).

 

[stockhausen]

At least ddr4 is cheap at the moment

Yeessss . . . unfortunately, as has been pointed out to me above, of no possible value to me at present

On a slightly different front, I get the impression that there should not be a problem with installing "faster" RAM in my system - I currently have 667 MHz planes installed. Looking on CPU-Z, 666 MHz planes are described as JEDEC #4 and run at a voltage level of 1.5. 1200 MHz described as XMP-2400 is mentioned and is supposed to run at 1.65 V - what on Earth is XMP-2400 SDRAM, does it exist and should I be able to use it?

More pertinently, is there any reason why I should not try something like DDR3 PC3-1866 planes (e.g. Crucial 2 x 8GB DDR3L-1866 UDIMMs)? Are they likely to be compatible and if not, would the system simply revert to 667 MHz?