One servo for the front, one servo for the rear. 4WD with one motor/trans in the middle. Skid steering doesnt seem like it will work. I read your post from here and will be investigating this option tonight:
_Sorry for the delay but the great news is this already works! Here's how.
Plug your 2nd, 3rd and 4th steering servo's into the pixhawk channel's 5,6 and 7. Then set RCnFUNCTION to 26 (ground steering) for each of those channels. So_
And now when you move the steering stick you should see those servo's move!
If they move in the wrong direction - say the one on channel 6 was moving in the wrong direction - then just reverse that channel by setting RC6REV to -1. If its already -1 for some reason just set it to 1._