Seems like I cant control 4 stepper motor at once
The variable name BR is already used somewhere else.
Use a different variable name
AccelStepper FL(1,stepPin,dirPin);
AccelStepper FR(1,stepPin2,dirPin2);
AccelStepper BL(1,stepPin3,dirPin3);
AccelStepper BackRight(1,stepPin4,dirPin4);