DDR4 is technically faster than DDR3.
To be more precise, it is capable of faster speeds.
Not all RAM are created equal, 4GB DDR3 won't achieve the same performance than 4GB DDR4 in most cases (however it is possible).
As DDR4 is capable of running at higher MHZ speeds, it's able to perform at a higher level than DDR3 of identical GB RAM size.