Hi, I had gone through your question and understand your concerns but your question is too late , I mean your baby is now 4 years old , has your baby received DPT boosters of one and half year , if not best option is to take a quadrovax (DPT plus Hib ) , you have not mentioned about completion of Hepatitis B series , it that is incomplete baby can be given a shot of pentavac now . If your baby received DPT booster at one and half year he can be given only HIB vaccine single dose . Your baby was protected by taking pentavalent but immunity has weaned of due to lack of completion of series and booster doses , so he can be protected in future by boosting immunity by taking single doose of HIB alone or HIB plus DPT (Quadrovax) , or HIB +DPT +HB(Pentavac ) depending upon status of other vaccines taken by your baby . Hope this answers your question. If you have additional questions then please do not hesitate in writing to us. Wishing your baby good health. •     DISCLAIMER: - All the information provided here is for information purpose only , it is not a substitute for the advice of a physician after physical examination , it is not intended to replace your relationship with your doctor. This information in no way establishes a doctor-patient relationship. Before acting anything based on this information do consult your doctor. I recommend that online users seek the advice of a physician who can perform an in-person physical examination