Something new that you'll notice in going from Monster Hunter 3 Ultimate to 4 Ultimate is that your companions, Cha-Cha and Kayamba are not around. Instead you will be accompanied by creatures that you will recognize as Felynes. For Monster Hunter 4 Ultimate, they are renamed as "Palicoes", and executive producer Ryozo Tsujimoto explains why:
The support Felynes are being rebranded as 'Palicoes' this time around and they don't have such a central story aspect, they'll be involved a little bit but not as much as Cha-Cha and Kayamba were. They're part of a system where you can bring these support characters with you out on quests; you can bring two of them with you with one being yours that you can customise, and the other one can be a different one, perhaps one you met on StreetPass from another player and you can even collect them. They're very much involved in the gameplay but not so much a story element this time. The differences, if you like, were that Cha-Cha and Kayamba were unique characters, and the Palicoes are kind of in a category of their own rather than being a character. You can enjoy collecting them, customising them and you can even rename them, for example my main support character is called Fujioka (MH4U creative director), and he gets to help me out on quests just like at work.
You can read the entire interview here on NintendoLife.