"The aims of this trial are to demonstrate the consistency of three manufacturing lots of GSK Biologicals' DTaP-IPV candidate vaccine in terms of immunogenicity and to evaluate non-inferiority of GSK Biologicals' DTaP-IPV vaccine with respect to immunogenicity and safety compared to the control vaccines (separate injections of GSK Biologicals' DTaP vaccine [Infanrix] and Aventis Pasteur's IPV vaccine [IPOL]) when administered as a 5th dose of DTaP and a 4th dose of inactivated poliovirus vaccine in subjects 4 to 6 years of age. Vaccines will be co-administered with the second dose of M-M-RII, which is recommended at this age. Concomitant administration of a US-licensed influenza vaccine will be allowed according to seasonal availability of vaccine and at the discretion of the investigator."