My DD was already at Preschool when she turned 4 and I never got her the booster because she had pneumonia at the time and it took 3 months to completely clear and I wasn't compromising her immune system any more than it already was and we had decided not to continue anyway. When it came time to enroll her at school late last year I told them that she didn't have the 4yo booster and they were fine with it. Legally they cannot exclude them for not having had it, but if there is an outbreak they will be required to stay at home for the duration.