I have heard many times that four-wheel drive is less efficient than two-wheel drive. Many car producers have a normal drive mode where only two wheels are powered, and turn the four-wheel drive on only when needed, to "save energy".
I have never understood why it should be so. You need to transfer only half the energy to each wheel (which must also save the material wear), so the total energy consumption must be the same.
Please answer the question also for the case of electric cars with four separate small engines each attached to a single wheel.