If a 1Fr 2Rr trike can turn in it's own length....
 

....why can't a four wheel front drive?

Something to do with CV joints not being so reliable at near 90 degree bends. That, and drive shafts tend to impact the tire itself when turned too sharply.

If you want a four-wheeled vehicle to turn inside its own footprint, you have to give it four-wheel steering and individually powered wheels.

I built a one wheel drive trike that can turn in under it's own length; the undriven wheel turns backwards. Of course it only turns that sharp in one direction but the other direction is really sharp too.

Yeah, I guess it would require wheel motors. And that would add unsprung weight.

Yeah, wheel motors, with all the unsprung weight that comes with that. Good for ATVs but not road cars.

Not fwd or 4ws but they sure can zero turn:

https://www.ruralking.com/media/cata...r_rz5424_1.jpg

With 4 wheels, I can't remind anything other than skidsteer tractors able to turn their own lenght.
http://2.bp.blogspot.com/-5yAW6ytedR...rregadeira.jpg

Not gonna happen, but I guess wagon steering would work. Still needs front wheel drive to work.

It's been done.
https://www.youtube.com/watch?v=4IpHQRKheac