The two-tier cost-cap regulations that Max Mosley tried to introduce included a provision for 4 wheel KERS harvesting, which I'm pretty sure was included as a way of opening the door for full four wheel drive ... I can't remember exactly what the regulations stated, but it must be an idea that is still on the table.
Fondtech have included 4 wheel drive in the spec for their newly revealed Formula REV car :
http://performancedrive.com.au/fondtech ... -revealed/
If this is likely to be introduced for electric vehicle racing (which the FIA is throwing its full weight behind) then it should be the shape of things to come in F1.