well a transfer box reduces the power to the front by a set ratio.....sending 1/2 the power to the front would probably make it drive funny...

Also if its open diff fwd gearbox then it'd be horrible if any of the 4 wheels broke traction...if it was a lsd box it'd also drive funny when traction btoke as the diff would lock possibly putting extra stress on things.

Thridly theyre normally weak when only driving 2 wheels the way they should..

I'm not sure if its a good idea really myself...if it was a simple setup i'd have seen one already

Hope that helped a bit and not just thrown spanners in the works..

Dan