The key to game 5 is whether the Diaw experiment continues to work. If Parker plays like he did in the first three games, it is a lost cause.
The sad truth is that when Hill re-injured himself in latter part of the first half of game 1, it destroyed the Suns game plan on both ends of the floor. Without Hill, Parker when wild and the Suns offense struggled. It was even worse in second half of game 2.
D'Antoni's experiement of starting Barbosa in game 3 was a disaster. It put Nash on Parker, which was hopeless and the Suns were effectively out of the game after 6 minutes. So while no one is going to stop Parker, the first three games showed just how important being able to limit him is:
Game 1 26 points on 11 of 20 shooting (most of it in the second half)
Game 2 32 points on 13 of 25
Game 3 41 points on 17 of 26
Game 4 18 points on 7 of 17
If the Suns can limit Parker without doubling him, they can beat the Spurs again.