I was just hoping that my daughter would enjoy the experience (that was the main goal). She loved it. Since I was a green skier at the time, I was around the entire day. By the middle of the first day, she was skiing down the beginner hill (albeit with the instructor skiing backwards bent over holding her tips, well worth every penny). By the end of the day, she could ski the hill.
I acknowledge that I speak out of ignorance for I don't snowboard, yet (probably 3-4 years before my daughter is interested). However, I would think that skiing may be easier to learn especially for a 4-5 year-old because of the more natural feet positioning (please don't flame me.)
A lot of work goes into making these kids programs as successful as they can be. I have personally taught kids younger than 5 to ski, but how well the child does is very dependent on the child. A timid five year old with seperation anxiety is a very different case than an independent and adventurous kid. I am not being judgemental here, it is just that every kid is different and some are ready at five while others are not.