Well, if your 4 year old, is anything like mine, she will LOVE cruising. My daughter loves wearing a party dress to dinner and eating in fancy restaurants .... so cruising is totally up her ally! She'll be five in February and weeks away from her second cruise. She loves it!
My daughter was okay with Camp Carnival. My son hated it. We spent most of the week with our kids, which was fine with us.
I'm guessing you won't find toooo many kids on Royal. While we love Royal, we find it expensive for the little ones.