NANKID OPTIPRO Step 3 & Step 4 are both formulated milk powders for children containing 2'-FL Oligosaccharide & Probiotic B. lactis to strengthen your child's immunity and reduce the likelihood of them feeling unwell.The only difference is that NANKID OPTIPRO Step 3 is for children ages 1 to 3 years old and above whereas NANKID OPTIPRO Step 4 is for children 4 years and above.