We actually had a total mechanism for this. If you look on our robot its 4 wheel drive but with a twist. There is a mechanism that is controlled by a motor that pushes a pair of castors onto the ground at the front, thus enabling much more manourveablility, at the expense of power. The idea was that you use 4wd to get the tetra to the goal and use the castors to align it more accurately. However the welds got smashed off during a nasty bump in one of the later rounds.
This pic is the best i can find.

If you look there there is a V shaped piece, which was later attached to the black motor. We then added a little flag that went up when the castors were down.