Ok, interesting. Issues may come up with the steering response if you want/expect the steering servo to turn the engines as well. in -rc4 you may find the steering servo does move (although I was planning to disable it in future releases if skid-steering is enabled).. but the scaling for speed will be incorrect. In a regular steering vehicle (like a car) the steering response needs to be scaled for speed because as the vehicle is moving more quickly, the response to a small steering input increases. With a skid-steer vehicle the response doesn't change as the vehicle speeds up.
Now, if the vehicle always tends to move at the same speed it may be possible to tune it to work OK.. but it's uncharted territory!