I've been taking lessons for about 6 months now. My instructor always reminds me to use my 4th finger. Also, he says playing out of the third position is good practice for hand and wrist position since it's kinda hard to flatten your hand out and still get at the G and D strings. On the other hand, I was listening to an instructional tape of Kevin Burke and he commented that Irish fiddlers tend to use a lot of open strings rather than the 4th finger so there you go. I agree with the others...learn both and use whichever is easiest for a given situation.
Dave T