In Japan, if the last thing to leave the ground, and the first thing to touch the ground are your feet (as opposed to any structure of the aircraft), you don't need a license...
4 engines? Because you can't do it with one - no pitch control on the rotor blades, so you can't handle autorotation. Two engines? They both need to be big enough to power the machine in case one fails. Three engines? A bit difficult to engineer, as they'd need to be bigger engines. Four engines? I just happen to have 4 of the right size engines lying around the workshop...
I wonder how yaw is controlled???