The 140 N-cm figure is for the motor, 3000 oz-in is for the output spindle. 1.4 N-m and 21 N-m in SI units.
Yes gears exchange torque for angular velocity whatever kind of motor you put them on!
Also how many steps are in 1 stepper motor rotation
Commonly 200, but you always read the datasheet.
and last of all, could a 428 oz-in or 1020 oz-in holding torque stepper motor lift a 4kg weight connected to the motor spindle 0.7m away from a vertical 115 degrees into the air.
Cannot understand that description. Perhaps a diagram?