3 + 1 refers to the up and down of two oil cylinders (2 axis), the back and forth movement of the reaer stop (1 axis), and the +1 axis generally refers to the crowning system. When the die is pressed down, the backgauge will give way or advance, so as to improve the accuracy of the product.
4 + 1 has one more rear stop, which can move up and down for positioning (double axis linear guide rail).
The price of these bending machines will be much higher when buying press brake machines. Servo motors, ball screws and linear guides all pay for precision.