I think I understand your question about configuring the 4th Axis with Mach3. You want the 4th Axis to read directly in Degrees.
Please note I am not an expert on MACH3 but I could not get my 4th axis to read directly in Degrees or Radians. However, there may be a way you can achieve this, and if so I hope one of the more informed Mach3 users will explain how it can be done.
This is how I use the 4th axis.
1) My axis uses a 200 step stepping motor, with a 10 micro step controller so it has 2000 steps per 1 revolution of the stepping motor.
2) The 4th Axis has a 40 :1 ratio, so 1 revolution of the 4th axis is
2000 x 40 = 80,000 steps per revolution (360 degress).
3) My CNC machine is metric and all the X, Y axis have ballscrews that move 5mm for 1 revolution of the stepping motor, and all the motor controllers use 10 micro steps. In the config motor tuning I set the steps per mm to be 400, and I use the same setting for the 4th Axis.
4) So 1 revolution of the 4th axis is 80,000/400 = 200mm, ie 360 degress is "200mm" or 200 units ! This may appear strange but it works well.
So if you want to move the 4th axis 1 degrees the actual value you move by is 1 x 200/360 = 0.555555mm. If you want 3.5 degrees = 3.5 x 200/360 =
If you want to use degrees as units it gets very difficult. For example for my 4th axis 2000 steps = 9 degrees so 1 degree would be 2000/9 = 222.2222 which is not an integer, you can't set the config motor tuning to 222.2222 per mm. However if you can find a stepping motor controller that you can set what ever micro step value you want you can do it.
You don't say what micro steps your Stepping Motor Controller can be set to, (I set mine to 10 for smooth movement and reasonable resolution). But lets say you can set it for 10, then for your 70 : 1 reduction you will have 10 X 200 x 70 = 140,000 steps per revolution (360 degrees). If you set the 4th axis in the config motor tuning the steps per mm to be 400, then you will have 350 mm (units) per 360 degrees. So just use the same calculations for say a movement of 3.5 degrees use 3.5 x 350/360 = 3.402778mm (units).
However, if you could get a stepping motor controller for 9 micro steps then you can do it. ie 9 x 200 x 70 = 126000 steps per revolution. Set the config motor tuning steps per mm to be 350, then 1 revolution would be 126000/350 = 360mm (or units degrees!)
The bottom line is that you just can't use the magic 360 units per rev unless you are very lucky with the gearing of the system and/or the microstepping features of the motor controller.
Sorry that this is a long reply but I hope it helps,