Physical milestones refers to your child's height and weight. Typically by the age of four, boys should be at around 33 to 39 pounds, and in height they should be about 39 to 41 inches tall. Girls are about 32 to 38 pounds and about 38 to 41 inches tall. While these are just ranges, and your child may fall below or over those ranges and there may not be cause for concern, it is still important to discuss this with your child's pediatrician to make sure that your child is developing on track.
While you may notice that in the playground or at school your child may be shorter or taller than others, and you may be concerned as to why that is, and you may think to yourself, "Well I don't recall being taller or shorter than anybody else. I feel that I was average. I don't know why my child is." It's OK. Kids develop at a different rate, and sometimes girls even develop faster than boys, typically, and they might even catch up to the boys.
So what's really important is that you're monitoring how your child is growing in terms of weight and height. That means speak with your pediatrician, and make sure that you have a copy of their growth chart so that you too can continue to monitor your child's growth and height.