I think both poll answers are sort of right. These are the four best players in the world, who else could you reasonably put there? Roddick? Davydenko? Nalabandian? Wawrinka? Ferrer? Murray, in my mind fits much better with the top three than any ot the other options.
Of course, Murray will need to win the tournament to make it a true 'big four', rather than a 'big three'.
Still, I think this was the best possible weekend result for all neutral fans, as there are four interesting and skilled players, and they have all (except Federer-Murray) had important matches against each other at some point in the season, and there will likely be a compelling storyline for the final no matter who advances.
I hope it'll be an excellent cap to a pretty solid two weeks.