Programming a Crab Drive System

[kamocat]

Idealy, you would want all four wheels independently rotated, otherwise you can't turn very fast when you're travelling diagonally (due to the sideways coeffecient of friction of the wheels).
If you care to do some soldering to wire up the keyangs, they are a possibility.
By the way, I would reccomend using a continuous-turn potentiometer to determine the wheel rotation. (Try to find one without a 1/8 turn blind spot).

Or, if you want to go fancy on the mechanical side, you could actually use a single motor for direction and a single motor for turn (using a differential on each wheel bracket to sum the two values).