This is a Phase 2, multicenter, randomized, partially double-blind study to evaluate the safety, tolerability, and immunogenicity of PG4 in healthy toddlers 12 through 15 months of age who previously received 3 infant doses of 20vPnC. This study will be conducted at investigator sites in the US and Puerto Rico. Approximately 225 children between 12 and 15 months of age will be enrolled and randomized in a 2:2:1 ratio by site-based randomization to receive either 1 or 2 doses of PG4 (90 participants per group) or 1 dose of the control vaccine 20vPnC (45 participants). The vaccine in toddlers randomized to receive 1 dose will be double-blind, as PG4 and 20vPnC have the same appearance and a single dose will be administered at Visit 101 of the study. For participants randomized to 2 doses of PG4, the parents/legal guardians and site staff will know that these participants will need to return for a second vaccination visit and that they will receive PG4 at both Visit 201 and Visit 202.