Possible to have different torque levels on MG966R servo with Rotary Potentiometer?

Generally it is not easy to control the torque of a standard servo, since mostly they aren't meant for that. A servo has an internal controller, which will read the input PWM signal and control the geared motor, so that the desired position will be reached. And to get there, the most servos will use the maximum torque. Also the torque of a servo motor depends on more variables, for example speed and type of load (See here for more information about this.

I read that by adjusting the input voltage, you can run the servo at different speed

That's not really true. Most servos only support a rather small voltage range, where they will work. You can get more torque, if you use a higher voltage, but it is not a simple linear relationship, since the torque depends on way more variables. One way may be to always drive the servo to it's current maximum torque and change this maximum torque by changing the input voltage inside the working range. Though this might not get you 4 torque levels, that are different enough for you.

Also wondering if changing angle stepsize of MG966R servo will affect torque in anyway?

It is unclear, what exactly you mean by "stepsize" here. If you mean to control the speed of the servo: Yes, it will affect the torque, but not in a linear way. And this greatly depends on the load, that you apply.

In the answer to this question, sensing the current of the servo is suggested as a measurement of (applied) torque. If you use a feedback loop with this, you might be able to control the (applied) torque. I haven't done this kind of thing, so I'm not sure, if this can really work.

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That said, you should again think about wether a normal servo is really the best way to go here, since it controls electronics, that you don't control yourself. For example you may use a geared DC motor, which you can directly control without any other electronics getting in your way.