I would suggest that your child learns to ride a bike without training wheels on pavement first and gets their confidence built up. Then, I would start them on riding over small features in the grass with a helmet and appropriate body armor because they are going to fall, a lot, at first. Again, they will need to build confidence. You can watch many You Tube videos on beginner MTB skills. I think sending them to any MTB camp at 4 years old would be a waist of money unless they are really good and passionate about mounting biking.