4-20 milliamp to volts to pressure maths
If I understand your question correctly, when the sensor is on,


4 mA (1 V on the Arduino) means 0 bar
20 mA (5 V on the Arduino) means 100 bar.


Is that correct?

If that is correct, there you just have to linearly map the voltage
range to the pressure range. The equation is

P = (v − 1 V) / 4 V × 100 bar

or, in C++:

float v = analogRead(A0) / 1024.0 * 5.0;  // in volts
float p = (v - 1) / 4 * 100;              // in bars