Felyne partners—now called Palicoes in Monster Hunter 4 Ultimate—now have more uses in battle. In addition to attacking monsters like they always could, they can now perform combination attacks with one another. In a new video, Capcom show this feature off, along with other Palico customization features.
Felynes also have stats in the game, and the combination attacks they will perform will depend on the kind of Felynes you have on you. You can read more about this and other Felyne features here.
Monster Hunter 4 Ultimate will be released in early 2015 for Nintendo 3DS.