The aim of this study is to assess the immune response, safety and reactogenicity following administration of an additional dose of a pneumococcal conjugate vaccine at approximately 4 years of age in children previously vaccinated with 3 primary doses of GSK 1024850A or Prevenar™ vaccine within the first 6 months of life and a booster dose of plain polysaccharide pneumococcal (Pneumovax 23™) vaccine at 11-14 months of age.