My first steps are power and motor selection.
Let's assume the arm is going to do 4ft and hold 20bs (say 10 + 10 pounds of arm) so I need 80 ft-lbs at the base.
I want 360 degree/sec so around 60rpm.
Power = torque * rpm = 80 ft-lbs *2*pi*60rpm /33000 so close to 1 HP (745W) at the base!
That's a lot of juice.
Most stepper motors are rated with a voltage and current. My understanding is that the voltage doesn't mean anything because you can drive it at much higher voltage as long as you meet the current requirements.
Anyway, let's look at something like the NEMA 23 PH266M which can be bought for around $20 on ebay and has 400steps/rev.
It's rated at 6V and 1.2A so does that mean it's only a 6W motor?