Rotary actuator and position sensing for hydraulics

I'd like to be able to flip between two pressures (LOW, 60 PSI and HIGH, 100 PSI) on a hydraulic regulator. I don't want to interfere with the actual hardware so I would like to make an actuator turn the knob just like a person would.

It needs to go from position A (LOW) to B (HIGH), or vice-versa when commanded.
Positions A and B are 90* apart.
When it is already in position A and gets a "LOW" command it should stay in place, same with B and a "HIGH" command, so it needs to know what position it is already in.
Would also need to be able to know what position it is in after its been turned off and back on.

Suggestions on how to accomplish this? Stepper and encoder? Stepper and limit switches?

My suggestion: DC Gearmotor and Limit Switches. The switch will tell the Arduino which position the knob is in. The Arduino would use a motor driver or a pair of relays to control the direction of the motor. You would need a power supply suitable for the motor and could use a Buck Converter to get 5V from that supply to provide power to the Arduino.

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And if on power-up, neither of the limit switches is on, you must decide before you program it, which way do you want the valve to go in order to find the limit switch.

I guess at power up there would already have to be a command to one position or the other so that would takes care of that.
Does it matter what kind of signal would control this, something like a +/-5v to determine HIGH/LO or two completely separate inputs?

It does matter. What is the source of the signal? Is it a contact closure, like a switch or relay? Is it an Open Collector (sometimes called 'NPN') or Open Drain output from a sensor? Is it an analog voltage with separate HIGH and LOW thresholds? In all cases, the input will need to be in the range 0V to 5V relative to the Arduino Ground.

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