To determine the age of rock #4, which is suggested to be between 700-750 million years old, you should use Uranium-238. This is because Carbon-14 is effective for dating relatively recent organic materials (up to about 50,000 years old) and is not suitable for very old rocks or sediment that are non-living.
Therefore, the correct choice is: 700-750 Million Years; Uranium-238; because rock is non-living.