Quad core is not twice as fast as the dual core is. While it sounds like since there is 4 as oppose to 2, that doesn't make it twice as fast. It's only a small step faster.
Well, kinda, sorta.
As with everything, it's all dependent. Mainly dependent on the application, and whether it's multi-threaded. For a single threaded process, you won't see any speed increase. For a true multi-threaded process (with a kernel that supports SMP), you will see close to double the performance. And even for processes that are single threaded, you can use process affinity to achieve vast performance boosts.
Now, you might not get double the performance during your desktop computing tasks, but that doesn't mean the quad core processor is only a small step faster, it just means you're not fully utilizing it.