You should clarify that you are using analog pins and not digital pins.
const int input1 = 3; // Base
const int input2 = 2; // Joint 1 (Close to base)
const int input3 = 1; // Joint 2 (Middle)
const int input4 = 0; // Joint 3 (End)
to.
const int input1 = A3; // Base
const int input2 = A2; // Joint 1 (Close to base)
const int input3 = A1; // Joint 2 (Middle)
const int input4 = A0; // Joint 3 (End)