4 Wheel Drive Motor Control

We have decided to use a 4 Wheel Drive System to control our rover. The code below are the function for front, back, left and right movement.

However, when I try testing the turning movement of the rover, the rover was unable to neither left nor right, and we have found out that the problem was the tires alignment problem, hence we have changed the 4 wheel drive control into a 2 wheel drive control. And after a few tries, I have changed the pulse width modulation(PWM) signal value for left and right motor control. The code will be shown below, with video attached.