If you refer to Growth Charts for Boys, your baby's weight is around 3 percentile(for 4-mth baby). Yes, a bit lighter. However, if it's still within the range of the chart, there should be no problem. Even if it's out of the range, if he can still have growth in length and head circumference, normally doctors won't consider it as any negative signs. Apart from the food he is taking, you may also check if he can sleep well. Good quality of sleep is very important too. Yes, you can also reconsider if the current brand of the milk powder is suitable for him. Actually, the ingredients of any brands of milk powder are more or less the same. However, sometimes babies may have different reactions.