Does the Xbox 360 250GB model also have the 4GB flash memory?

Presumably, the 4GB being flash would be faster then the mechanical 250GB drive. So is the 250GB a straight swap out for the 4GB losing its advantage, or is the 250GB an ADDITION keeping the 4GB for actual memory purposes and faster loading. OR am I just totally over yzing this. I want an optimal system to be good for awhile.