Experiment here:

http://*.qq.com/tool/huoying/mnq.htm

This is mostly to due to Sasuke and Hinata and Lightning's chidori having crazy synergy.

The fourth character can actually be a lot of things. Neji, Choji, Karin, Kabuto, they all feature crazy combo potential with different utility. Choji has 8-hit combos just like Karin but features another Knockdown targetable jutsu that triggers the combo. So if you picked Choji, you can have three powers that start this combo when you want to murder someone. I call it single target assassination.

If you use Neji he has only 7 combos but he's really good at silencing people and can tank by himself on the side instead of leaving Sasuke or your Main vulnerable. Meanwhile Karin and Kabuto have obvious advantages, Karin being the more offensive healer while Kabuto spams clones.