5-dose series at 2, 4, 6, 15–18 months, 4–6 years

Prospectively: Dose 4 may be administered as early as age

12 months if at least 6 months have elapsed since dose 3.

Retrospectively: A 4th dose that was inadvertently

administered as early as 12 months may be counted if at least 4 months have elapsed since dose 3.

https://www.cdc.gov/vaccines/schedules/downloads/child/0-18yrs-child-combined-schedule.pdf