What is the difference between Fulcrum Racing 4 and racing 6 wheels? I know that specialized uses a custom wheel from fulcrum racing that leaves them with a unique numbering system not found on the Fulcrum site. 

Backstory is that i have 4's on one bike and 6's on another. As I plan to sell the older of the two bikes I want to keep the better of the two wheel sets.
Theoretically, the smaller the number - the higher up the Fulcrum series.

Fulcrum produce OEM wheels for manufacturers and these are supplied to manufacturers with "even" numbers which do not appear on the manufacturer's site. The number falling between the official numbering series usually indicates the wheel being an amalgamation of wheel series. This would be something like a 3 series rim with a 5 series hub and so on. The other point with OEM products is they are often made on production lines outside of the mfrs normal line of products.