DDR3 and DDR4 are not compatible because they are different in both pin count, arrangement, and signal definitions. In addition, locations of fool-proof notches on the golden fingers are different, so both are not compatible.
Though the DDR3 module is developed based on the DDR2 and also uses the 240-pin design, both the pin layout and signal definition of the DDR3 module are different. The difference is also seen in the fool-proof cut on the gold finger. Therefore, they are not compatible. Moreover, the DDR3 has enhanced I/O buffer performance by means of the 8-bit prefetch technology. Therefore, it can break through the clock speed limit of the DDR2 module. As a result, the data rate of the DDR3 module begins from 800MHz, and it will increase to DDR3-1066, DDR3-1333, and DDR3-1600 in the future.