ujm,
two notes:
1) this is for wuhan vaccine v. omicron, and large number of antibodies are needed to defend against a different variant as each antibody/vaccine encounter is less successful. Omicron vaccines are currently in phase 3 testing.
2) people generally acquire long-term immunity (T-cells, etc – beyond antibodies circulating in blood) to coronaviruses by encountered them multiple times during childhood. That is, we still get infected by “common cold” but with no serious damage.
So, presumably and logically, every encounter with either covid or a vaccine moves us towards that goal. According to many, that now include CDC, these encounters need to have some time between them, like two months. You surely want first several encounters to be vaccine and not covid to avoid possible damage. British results on side effects show 2 case of myocarditis/million for both Pfizer 2nd dose and booster, so no increased risk. So, boosters seem to be not controversial at least for population 40+ y.o. So, 4 months is too much, but presumably 6-12 month separate boosters (esp if tuned for the current variant) will make most of the adults fully safe from the “endemic” covid.