Pasto,
IMHO 4 days is really short.... better than nothing maybe, but way too short. It will take the better part of a day (each way) once on the island just to get to Twillingate from P aux B. leaving you about 2 days to shoot. If you go to the northern peninsula, you won't even have that. If you get a couple of soupy days, you won't have anything. When you count the drive/en route accomodation/ferry, etc that's going to be an expensive trip. You may want to consider flying Montreal to Deer Lake and renting a car from there. That will about double your days on the ground and increase your odds for not much more money when you consider everything.
Any current or retired Canadian Forces members going to Newfoundland should be aware that the ferry company has a great deal for you... free passage for you and your party (up to 4). It doesn't include the vehicle portion of the fare, but can still save you big bucks depending on the size of your party. They don't advertize this fare and you can't get it by booking on the web. You need to call the 1-800 number and ask for it.