Well, the Suns are alive for another day after beating the Spurs handily. Being down 3-1 is quite a mountain to climb, especially considering that they just won their first game of the series.
If the Mavs lose tonight, they will find themselves in a similar hole, but will have paced it differently with a Game 3 victory instead of Game 4. But really, what's the difference? You still have to win three straight against a team that has already bested you 75 percent of the series thus far.
So, can you call this a must win? With the steaming pile they ran out on the floor in New Orleans, you have to think that their odds heading back there for Game 5 aren't great. They won't be getting the calls like they've been getting here, and the Hornets are certainly more on their game there than in Dallas. So, in that respect, yes, it is a must-win.