Isn't that a standard carrier piece? It says 2 snowboards + 2 pairs of ski's with poles.
http://www.bmw.com/com/en/owners/acc...l?proDsId=4622
Might be able to get 4 in there might not.. so you may have better luck with Thule or something.
4 snowboarders in a 1 series probably not very comfortable though right?