Programming Direct drive with Direct (car like steering)

[tuna.and.cheese]

Hi everyone. I am trying to implement direct drive where all four motors are connected directly to all four wheels. I also want these wheels to each turn like a car would to make turns. If the joystick is in the upper left position, the robot will turn left and go forward and if in the right position, the same but turn right. The drive train is already built and the electronics are on the robot, but I am yet to attempt to program this and am just in the research stage.

Would anyone know how I could go about attempting this? I could get the robot to go foward and back, but to do this while at the same time rotating motors to turn left or right is not making too much sense.

Any help will be great. Thanks. Justin.