They don't have to win. They could be outmatched in a 4 vs 1 and just do enough to delay the Konoha shinobi for a while and then escape like they did the last time vs Jiraiya.kisame and itachi are going to win their matches.....
not to say that anyone will die though, i doubt we'll see any deaths between these 2 matches, but we all know itachi can't lose, and i think kisame is due for some more recognition.
Obviously Itachi and Kisame aren't going to die since they have bigger roles in the story to be cut off now. Other than Sasuke's storyline with Itachi, they are still channeling that jutsu to extract the yoma. If they are killed then Ataksuki won't be able to complete the jutsu and it kills the dramatic race to save Gaara.
But it is likely that Kisame and Itachi will be defeated here. Remember, they are using Kage Bunshin to channel the jutsu. So as they are right now, they aren't nearly as strong as they would be under normal circumstances. So either they will be realize that they outmatched and then retreat, or it will turn out that Konoha is fighting Kage Bunshins and they will think they have killed them only to have Itachi's and Kisame's bodies go up in a puff of smoke.