When DTaP-IPV / Hib is used for the 4-dose schedule (at ages 2, 4, 6, and 15 to 18 months), an additional booster dose of IPV-containing vaccine (IPV or DTaP-IPV) should be given at age 4 to 6 years, resulting in a 5-dose schedule; however, DTaP-IPV / Hib should not be used for the booster dose at age 4 to 6 years.