Open practice occurred for 2WD as it was easier for the organisers, who were still preparing things that morning, we only have so many hands to help. (I think I was outside the park putting up posters etc. for example when practice got underway).
It was discussed on the Saturday night that it was one of the shortcomings of the day, and so (with Chris Stewarts emphasis on fair practice for everyone), he controlled all of the timed practice on the Sunday for 4WD.