xespack has the factory description.... but to most aftermarket parts makers/mechanics 2v refers to 'small port' and 4v refers to 'large port' heads.
In the clevelands, the 4v heads are so big that most people use 2v's with 4 barrell carbs to get some form of low rpm performance out of the engines. There is also open and closed chamber versions of both heads available too.
As far as BB ford engines go... i'm just learning myself :)
Just like a Tsunami.