What is the difference between Fulcrum Racing 4 and racing 6 wheels?
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.