What the right dose of miralax for a 4 month old baby?

I recommend 2.5-5 ml: but this depends a little on the size of your baby. Preschool children easily tolerate 1 cap full, which is 17 grams (about a tablespoon or 15ml). If dose is not effective in 8-12 hours, a repeat dose may be given until stool is soft. Too large a dose will cause diarrhea and can potentially cause dehydration, so be sure to offer fluids.