Height and weight should be fairly proportional but what *your* child should be specifically is dependent on your child.
Developmentally, the range for 'normal' milestone markers is wide. For example, when your baby is older, learning to walk within a 'normal' time frame is anywhere from about 10 months to 17 months. This means when you see the "your child should/may" lists, keep in mind they're approximate and yours may do them earlier or later and still be within 'normal' ranges. If you're worried, as your doctor. At 4 months, your baby maybe laughing, smiling, making more eye contact that before, lifting head/shoulders from ground during tummy time, experimenting with babbling sounds and new baby noises, grasping objects (try a rattle), rolling-over.
Again, these are approximate and will vary dependent on child.
at 9:55 AM on Jun. 28, 2010