Basically, it does mean 4gh, only a bit differnt 
In order for you to get 4ghz potential from a dual proccessor board, the application has to be whats called "Multi-Threaded"
This is a program that is aware of the dual proccessor setup and can take advantage of it. Your Operating system will if it is win 2k or higher, or any version of Linux.
And many high end graphical programs and video editing, etc do as well.
Biggest downfall is that you wont notice "much" of a difference in gaming.
In order for you to get 4ghz potential from a dual proccessor board, the application has to be whats called "Multi-Threaded"
This is a program that is aware of the dual proccessor setup and can take advantage of it. Your Operating system will if it is win 2k or higher, or any version of Linux.
And many high end graphical programs and video editing, etc do as well.
Biggest downfall is that you wont notice "much" of a difference in gaming.