this was talked about else where on the forums, I believe a race bike build thread when someone had different year wheels.
from what i recall the Gen 4 can use a front wheel from the Gen 2s and up. The rear wheel cant be used from any previous years without spacers used. I believe the axle is a different size and youd need to accommodate for that. It has been done though.
I am sure someone else with more knowledge will chime in as this is just what I remember from someone else doing the work.