For the Republicans, those four will be Giuliani, Romney, Thompson and maybe Huckabee. Although I realize he's ahead of my man, Fred, in the polls, I just don't think McCain has the temerity to stick with it to the convention. He looks tired - whether it's of the campaign or politics in general I can't say.
Among the Democrats, I think it's obvious that the big three will be Clinton, Obama and Edwards. I don't really know too much about that side of the race - it's kind of like paying attention to only the American or National League during baseball season. But what I do know says these three appear to be the only real contenders.
If you are interested, there is a website that appears to give a comprehensive and fairly impartial listing of the candidates positions on a wide variety of important topics. Go to http://www.issue2008.com/.
It's good reading.
Be well and remember to vote!