Antibody levels are often cited as prediction surrogates early on because they are easy to measure. Occurrence or recurrence of infection, irrespective of antibody levels, of course, takes time. Also remaining to be seen is the role of T-cell mediated immunity in reducing the severity of Covid infection. If that role is significant, then declining antibody levels may be less worrisome than people believe currently.
...but that's also not the only type of immune response that a vaccine generates.
Vaccines cause the creation of b-cells, which retain the memory of this virus antigen and will re-create antibodies if the virus infects the host.
b-cell immunity is persistent - for years, and probably decades. But because it is very hard to measure (it resides in your bone marrow), it is therefor not often reported on.
Having said that, it is hard to imagine that a 4th booster significantly increases the number of anti-covid b-cells - let alone ones that can impart immunity against the newer variants.
... Which is expressly advised against by the EMA, as linked in the article.
So basically it doesn’t confer any long term benefits - the only value from it would be if you want to keep your circulating antibodies high by perpetually boosting on a regular interval.